MOSCOW/GENEVA (Reuters) -- President Vladimir Putin announced out of the blue on Monday that "the main part" of Russian armed forces in Syria would start to withdraw, telling his diplomats to step up the push for peace as U.N.-mediated talks resumed on ending the five-year-old war.
Damascus rejected any suggestion of a rift with Moscow, saying President Bashar al-Assad had agreed on the "reduction" of Russian forces in a telephone call with Putin.
Western diplomats speculated that Putin may be trying to press Assad into accepting a political settlement to the war, which has killed 250,000 people, although U.S. officials saw no sign yet of Russian forces preparing to pull out.
The anti-Assad opposition expressed bafflement, with a spokesman saying, "Nobody knows what is in Putin's mind".
Russia's military intervention in Syria in September helped to turn the tide of war in Assad's favor after months of gains in western Syria by rebel fighters, who were aided by foreign military supplies including U.S.-made anti-tank missiles.
Putin made his surprise announcement, that came with no advance word to the United States, at a meeting with his defense and foreign ministers.
Russian forces had largely fulfilled their objectives in Syria, Putin said. But he gave no deadline for the completion of the withdrawal and said forces would remain at a seaport and airbase in Syria's Latakia province.
In Geneva, United Nations mediator Staffan de Mistura told the warring parties there was no "Plan B" other than a resumption of conflict if the first of three rounds of talks that aim to agree a "clear roadmap" for Syria failed to make progress.
Putin and U.S. President Barack Obama spoke by phone on Monday about Syria, with the Kremlin saying the two leaders "called for an intensification of the process for a political settlement" to the conflict.
The White House said Obama welcomed the reduction in violence since the beginning of the cessation of hostilities but "underscored that a political transition is required to end the violence in Syria."
A U.S. official, speaking on condition of anonymity, said Washington was encouraged by Putin's announcement but that it was too early to say what it means, whether he will carry it out and what may have motivated it.
Putin said at the Kremlin meeting that he was ordering the withdrawal from Tuesday of "the main part of our military contingent" from the country.
"The effective work of our military created the conditions for the start of the peace process," he said. "I believe that the task put before the defense ministry and Russian armed forces has, on the whole, been fulfilled."
With the participation of the Russian military, Syrian armed forces "have been able to achieve a fundamental turnaround in the fight against international terrorism", he added.
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Putin says Russians to start withdrawing from Syria, as peace talks resume
Migrants protest behind a fence against restrictions limiting passage at the Greek-Macedonian border, near Gevgelija, on December 1, 2015. Since last week, Macedonia has restricted passage to northern Europe to only Syrians, Iraqis and Afghans who are considered war refugees. All other nationalities are deemed economic migrants and told to turn back. Macedonia on November 29 finished building a fence on its frontier with Greece becoming the latest country in Europe to build a border barrier aimed at checking the flow of migrants. / AFP / ARMEND NIMANI (Photo credit should read ARMEND NIMANI/AFP/Getty Images)
RIGONCE, SLOVENIA - OCTOBER 23: Migrants are escorted through fields by police as they are walked from the village of Rigonce to Brezice refugee camp on October 23, 2015 in Rigonce,, Slovenia. Thousands of migrants marched across the border between Croatia into Slovenia as authorities intensify their efforts to attempt to cope with Europe's largest migration of people since World War II. (Photo by Jeff J Mitchell/Getty Images)
AYDIN, TURKEY - JANUARY 22: Turkish Coast Guard personnel help refugees as they swim with buoys, after they toppled over a fishermen dinghy en route to Greece on January 22, 2016 in Didim district of Aydin, Turkey. 4 refugees body including 3 children pulled out of the water as Turkish coast guard personnel rescued 43 others. (Photo by Emin Menguarslan/Anadolu Agency/Getty Images)
TOVARNIK, CROATIA - SEPTEMBER 20: Migrants desperately try and board a train heading for Zagreb from Tovarnik station on September 20, 2015 in Tovarnik,Croatia. Croatia continues to send buses and trains north to its border with Hungary, as officials have estimated the around 20,000 migrants have entered since Wednesday. (Photo by Jeff J Mitchell/Getty Images)
SKALA SIKAMINIAS, GREECE - OCTOBER 03: A woman holds her child as she arrived with other migrants on the shores of the Greek island of Lesbos after crossing the Aegean sea from Turkey on an fishing boat on October 3, 2015 near village of Skala Sikaminias, Greece. Despite bad weather due to the upcoming Autumn, migrants and refugees are risking their lives in search of a better one in the European Union. Officals have warned that a rise in migrant deaths is expected as weather conditions gradually worsen. (Photo by Matej Divizna/Getty Images)
HEGYESHALOM, HUNGARY - SEPTEMBER 22: Hundreds of migrants who arrived by train at Hegyeshalom on the Hungarian and Austrian border walk the four kilometres into Austria on September 22, 2015 in Hegyeshalom, Hungary. Thousands of migrants have arrived in Austria over the weekend with more en-route from Hungary, Croatia and Slovenia. Politicians across the European Union are holding meetings on the refugee crisis with EU leaders attending an extraordinary summit on Wednesday to try and solve the crisis and the dispute of how to relocate 120,000 migrants aross EU states. (Photo by Christopher Furlong/Getty Images)
A member of the Macedonian police force and a migrant hold an injured boy during a clash between Macedonian police forces and migrant trying to cross an illegal crossing point on the border between Greece and Macedonia near the town of Gevgelija on August 22, 2015. Hundreds of mostly Syrian refugees forced their way over the Macedonian border today as police hurled stun grenades in a failed bid to stop them breaking through, an AFP reporter said. AFP PHOTO / ROBERT ATANASOVSKI (Photo credit should read ROBERT ATANASOVSKI/AFP/Getty Images)
A Turkish police officer stands next to a migrant child's dead body off the shores in Bodrum, southern Turkey, on September 2, 2015 after a boat carrying refugees sank while reaching the Greek island of Kos. Thousands of refugees and migrants arrived in Athens on September 2, as Greek ministers held talks on the crisis, with Europe struggling to cope with the huge influx fleeing war and repression in the Middle East and Africa. AFP PHOTO / Nilufer Demir / DOGAN NEWS AGENCY
= TURKEY OUT = (Photo credit should read Nilufer Demir/AFP/Getty Images)
CELLE, GERMANY - OCTOBER 27: A young Afghan girl plays outside tents prior moving into a weather-resistant accommodation facility at a shelter for asylum-seekers on October 27, 2015 in Celle, Germany. Many of the tent camps built across Germany to house the massive and recent influx of migrants are inadequate for the coming winter and authorities are scrambling to provide more durable accommodation. Germany has already registered 800,000 migrants this year and is struggling to handle the current flood, which in recent weeks has totaled up to 10,000 per day, mostly migrants arriving via the Balkan route through Austria into Bavaria. (Photo by Alexander Koerner/Getty Images)
KOS, GREECE - AUGUST 31: Migrants from Pakistan land on shore after completing a journey in a small dinghy crossing a three mile stretch of the Aegean Sea from Turkey August 31, 2015 in Kos, Greece. Migrants from many parts of the Middle East and African nations continue to flood into Europe before heading from Athens, north to the Macedonian border. Since the beginning of 2015 the number of migrants using the so-called 'Balkans route' has exploded with migrants arriving in Greece from Turkey and then travelling on through Macedonia and Serbia before entering the EU via Hungary. The number of people leaving their homes in war torn countries such as Syria, marks the largest migration of people since World War II. (Photo by Dan Kitwood/Getty Images)
SIKAMINIAS, GREECE - OCTOBER 14: People are assisted moments after they arrive with other Syrian and Iraqi refugees on the island of Lesbos from Turkey on October 14, 2015 in Sikaminias , Greece. Dozens of rafts and boats are still making the journey daily as thousands flee conflict in Iraq, Syria and Afghanistan. More than 500,000 migrants have entered Europe so far this year. Of that number four-fifths of have paid to be smuggled by sea to Greece from Turkey, the main transit route into the EU. Nearly all of those entering Greece on a boat from Turkey are from the war zones of Syria, Iraq and Afghanistan. (Photo by Spencer Platt/Getty Images)
SUBOTICA, SERBIA - SEPTEMBER 09: Migrants make their way through Serbia, near the town of Subotica, towards a break in the steel and razor fence erected on the border by the Hungarian government on September 9, 2015 in Subotica, Serbia. Thousands of migrants have funnelled their way across country to the small gap in the steel fence unopposed by the authorities. Since the beginning of 2015 the number of migrants using the so-called 'Balkans route' has exploded with migrants arriving in Greece from Turkey and then travelling on through Macedonia and Serbia before entering the EU via Hungary. The number of people leaving their homes in war torn countries such as Syria, marks the largest migration of people since World War II. (Photo by Christopher Furlong/Getty Images)
A migrant, exhausted, rests on the beach after failing to board a boat to the Greek island of Kos on early August 16, 2015 off the shore of Bodrum, southwest Turkey. Authorities on the island of Kos have been so overwhelmed that the government sent a ferry to serve as a temporary centre to issue travel documents to Syrian refugees -- among some 7,000 migrants stranded on the island of about 30,000 people. The early hours are the safest time for migrants travelling from Turkey to the Greek islands just across the water, which have seen a huge influx of refugees escaping the civil war in Syria and chaos in Afghanistan since the beginning of this year. AFP PHOTO/BULENT KILIC (Photo credit should read BULENT KILIC/AFP/Getty Images)
A young girl holding a stuffed animal crosses railway tracks in front of Macedonian police officers facing migrants demonstrating as they try to get across the border, after she crossed Greek-Macedonian border near Gevgelija along with other migrants and refugees on November 22, 2015. Serbia and Macedonia, which lie on the main migrant route to northern Europe, have begun restricting the entry of refugees to just those from certain countries, the UN refugee agency said on November 19. AFP PHOTO / ROBERT ATANASOVSKI (Photo credit should read ROBERT ATANASOVSKI/AFP/Getty Images)
A young migrant's hair becomes stuck while crawling under a barbed fence with her family at the Hungarian-Serbian border near Roszke, on August 27, 2015. As Europe struggles with its worst migrant crisis since World War II, Hungary has become, like Italy and Greece, a 'frontline' state. So far this year, police say around 141,500 migrants have been intercepted crossing into Hungary, mostly from neighbouring Serbia. AFP PHOTO / ATTILA KISBENEDEK (Photo credit should read ATTILA KISBENEDEK/AFP/Getty Images)
Refugees and migrants sail towards the Greek island of Lesbos on October 25, 2015 as they cross the Aegean sea from Turkey. At least three migrants -- two children and a woman -- drowned when their boat sank off the Greek island of Lesbos, the coastguard said, the latest fatalities in Europe's refugee crisis. Around a dozen others, mostly Afghans, are still missing after the rickety vessel, carrying 60 people, went down at dawn as it made the perilous crossing from Turkey, according to the Greek coastguard. AFP PHOTO / ARIS MESSINIS (Photo credit should read ARIS MESSINIS/AFP/Getty Images)
RIGONCE, SLOVENIA - OCTOBER 23: Migrants walk from the village of Rigonce to Brezice refugee camp in tha dark on October 23, 2015 in Rigonce, Slovenia. Thousands of migrants marched across the border between Croatia into Slovenia as authorities intensify their efforts to attempt to cope with Europe's largest migration of people since World War II. (Photo by Jeff J Mitchell/Getty Images)
A child is passed over a fence as migrants and refugees prepare to board a train heading to Serbia from the Macedonian-Greek border near Gevgelija on October 25, 2015. European Union and Balkan leaders hold emergency talks on Europe's refugee crisis amid threats from three frontline states to close their borders if northern EU countries stop accepting migrants. The mini summit, called by European Commission President Jean-Claude Juncker, groups the heads of 10 EU nations, including German Chancellor Angela Merkel, plus the leaders of Albania, Serbia and Macedonia. AFP PHOTO / ROBERT ATANASOVSKI (Photo credit should read ROBERT ATANASOVSKI/AFP/Getty Images)
CALAIS, FRANCE - AUGUST 3 : A migrants camped out in Calais, attempts to climb a security fence near refugee camp of Calais, France on August 3, 2015. More than 2 thousands migrants waiting at Calais forests, hoping to get to Britain, camp out in Calais, on August 3, 2015. (Photo by Mustafa Yalcin/Anadolu Agency/Getty Images)
MYTILENE, GREECE - OCTOBER 22: Afghan men argue outside of the main gate as violence escalates for migrants waiting to be processed at the increasingly overwhelmed Moria camp on the island of Lesbos on October 22, 2015 in Mytilene, Greece. Dozens of rafts and boats are still making the journey daily as thousands flee conflict in Iraq, Syria, Afghanistan and other countries. More than 500,000 migrants have entered Europe so far this year. Of that number, four-fifths have paid to be smuggled by sea to Greece from Turkey, the main transit route into the EU. Nearly all of those entering Greece on a boat from Turkey are from the war zones of Syria, Iraq and Afghanistan. (Photo by Spencer Platt/Getty Images)
DOBOVA, SLOVENIA - OCTOBER 22: Migrants are escorted by police through Dobova as they walk to a holding camp on October 22, 2015 in Dobova, Slovenia. Thousands of migrants marched across the border between Croatia into Slovenia as authorities intensify their efforts to attempt to cope with Europe's largest migration of people since World War II. (Photo by Jeff J Mitchell/Getty Images)
MIRATOVAC, SERBIA - JANUARY 27: Migrants walk through thick mud as they cross the Macedonia / Serbia border in the southern Serbian village of Miratovac on January 27, 2016 in Miratovac, Serbia. Migrants have been braving sub zero temperatures as they cross the border from Macedonia into Serbia. (Photo by Milos Bicanski/Getty Images)
Migrants walk in the rain after crossing the Greek-Macedonian border, near Gevgelija, on November 27, 2015. Since last week, Macedonia has restricted passage to northern Europe to only Syrians, Iraqis and Afghans who are considered war refugees. All other nationalities are deemed economic migrants and told to turn back. Some 800 people are stuck on the border, mostly Iranians, Moroccans, Bangladeshis and Pakistanis. / AFP / ROBERT ATANASOVSKI (Photo credit should read ROBERT ATANASOVSKI/AFP/Getty Images)
CALAIS, FRANCE - DECEMBER 01: Migrants contend with wintery conditions in the camp known as the 'New Jungle' on December 1, 2015 in Calais, France. Thousands of migrants continue to live in the makeshift camp in the port town in northern France, where they continue to try and board vehicles heading for ferries or through the tunnel in an attempt to reach Britain. (Photo by Jeff J Mitchell/Getty Images)
TOPSHOT - Syrian refugees are pictured in a camp as Syrians fleeing the northern embattled city of Aleppo wait on February 6, 2016 in Bab al-Salama, near the city of Azaz, northern Syria, near the Turkish border crossing.
Turkey on Saturday said it was expecting a huge wave of Syrians fleeing a government onslaught on rebel-held territory, with a regional governor saying at least 70,000 people may be heading for the border. The United Nations said some 20,000 people had gathered at Syria's Bab al-Salam crossing with Turkey. An AFP reporter said the crossing was closed but the Syrian side of it was being supplied by aid trucks coming from Turkey.
/ AFP / BULENT KILIC (Photo credit should read BULENT KILIC/AFP/Getty Images)
DEREK, SYRIA - NOVEMBER 13: Yazidi refugees celebrate news of the liberation of their homeland of Sinjar from ISIL extremists, while at a refugee camp on November 13, 2015 in Derek, Rojava, Syria. Kurdish Peshmerga forces in Iraq say they have retaken Sinjar, with the help of airstrikes from U.S. led coalition warplanes. The Islamic State captured Sinjar in August 2014, killing many and sexually enslaving thousands of Yazidi women. (Photo by John Moore/Getty Images)
GEVGELIA, MACEDONIA - AUGUST 23: Macedonian special police forces control the departure of migrants to board a train to reach the Serbian Macedonian border on August 23, 2015 in Gevgelia, Macedonia. Thousands of migrants and refugees have arrived on the border between Greece and Macedonia with many trying to travel through Macedonia to reach northern Europe. Macedonian security forces are expected to let several hundred migrants board trains towards Serbia and the rest of Europe. (Photo by Milos Bicanski/Getty Images)
LETSCHIN, GERMANY - OCTOBER 09: Mohamed Zayat, a refugee from Syria, plays with his daughter Ranim, who is nearly 3, in the one room they and Mohamed's wife Laloosh call home at an asylum-seekers' shelter in Vossberg village on October 9, 2015 in Letschin, Germany. The Zayats arrived approximately two months ago after trekking through Turkey, Greece and the Balkans and are now waiting for local authorities to process their asylum application, after which they will be allowed to live independently and settle elsewhere in Germany. Approximately 60 asylum-seekers, mostly from Syria, Chechnya and Somalia, live at the Vossberg shelter, which is run by the Arbeiter-Samariter Bund (ASB) charity. Vossberg village is located in rural eastern Germany close to the border to Poland, and unlike shelters in southeastern Germany, it has experienced no incidents of right-wing animosity from locals, something an ASB spokesman attributes to strong cooperation between the municipality, schools and citizens' groups and an effective information campaign to educate locals about the newcomers. Germany has been inundated with hundreds of thousands of asylum applicants this year and is struggling to accommodate them. (Photo by Sean Gallup/Getty Images)
ROTHESAY, ISLE OF BUTE, SCOTLAND - DECEMBER 04: Syrian refugee families arrive at their new homes on the Isle of Bute on December 4, 2015 in Rothesay, Isle of Bute, Scotland. The Isle of Bute is welcoming 15 Syrian Refugee families as part of the governments plant to give refuge to 20,000 refugees in the UK by 2020. The Isle of Bute, on the Cowal peninsular, has a population of 6,498 which swells in the Summer months due to tourism. The island has been nicknamed the 'Madeira of Scotland' (Photo by Christopher Furlong/Getty Images)
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Kremlin spokesman Dmitry Peskov said Putin had telephoned the Syrian president to inform him of the decision, but the two leaders had not discussed Assad's future, the biggest obstacle to reaching a peace agreement.
The move was announced on the day U.N.-brokered talks involving the warring sides in Syria resumed in Geneva.
In Damascus, the Syrian presidency said in a statement that Assad had agreed to the reduction in the Russian air force presence, and denied suggestions it reflected a difference between the two countries
"The whole subject happened in complete coordination between the Russian and Syrian sides, and is a step that was carefully and accurately studied for some time," the statement said, adding that Moscow had promised to continue support for Syria in "confronting terrorism."
Syria regards all rebel groups fighting Assad as terrorists.
Rebels and opposition officials alike reacted skeptically.
"I don't understand the Russian announcement, it's a surprise, like the way they entered the war. God protect us," said Fadi Ahmad, spokesman for the First Coastal Division, a Free Syria Army group fighting in the northwest.
Opposition spokesman Salim al-Muslat demanded a total Russian withdrawal. "Nobody knows what is in Putin's mind, but the point is he has no right to be in be our country in the first place. Just go," he said.
A European diplomat was also skeptical. "It has the potential to put a lot of pressure on Assad and the timing fits that," the diplomat said.
"However, I say potentially because we've seen before with Russia that what's promised isn't always what happens."
MOMENT OF TRUTH
The Geneva talks are the first in more than two years and come amid a marked reduction in fighting after last month's "cessation of hostilities," sponsored by Washington and Moscow and accepted by Assad's government and many of his foes.
Russia's U.N. ambassador, Vitaly Churkin, confirmed some forces would stay in Syria. "Our military presence will continue to be there, it will be directed mostly at making sure that the ceasefire, the cessation of hostilities, is maintained," he told reporters at the United Nations in New York.
But he added, "Our diplomacy has received marching orders to intensify our efforts to achieve a political settlement in Syria."
Speaking before Putin's announcement, de Mistura said Syria faced a moment of truth, as he opened talks to end a war which has displaced half the population, sent refugees streaming into Europe and turned Syria into a battlefield for foreign forces and jihadis.
The limited truce, which excludes the powerful Islamic State and Nusra Front groups, is fragile. The warring sides have accused each other of multiple violations and they arrived in Geneva with what look like irreconcilable agendas.
The Syrian opposition has said the talks must focus on setting up a transitional governing body with full executive power, and that Assad must leave power at the start of the transition. Damascus has said Assad's opponents are deluded if they think they will take power at the negotiating table.
(Additional reporting by Phil Stewart, Arshad Mohammed, Michelle Nichols, Tom Miles, Tom Perry and Stephanie Nebehay; writing by Dominic Evans and David Stamp; editing by Peter Millership)