Russia has started flying drone aircraft on surveillance missions in Syria, U.S. officials said on Monday, in what appeared to be Moscow's first military air operations there since staging a rapid buildup at a Syrian air base.
The beginning of Russian drone flights underscored the risks of U.S.-led coalition planes and Russian aircraft operating within Syria's limited airspace, without agreeing on coordination or objectives in Syria's civil war.
The former Cold War foes have a common adversary in Islamic State militants in Syria. But Washington opposes Moscow's support for Syrian President Bashar al-Assad, seeing him as a driving force in the four-and-a-half year-long civil war.
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Russia starts Syria drone surveillance missions: U.S. officials
SOCHI, RUSSIA- SEPTEMBER 17: Russian President Vladimir Putin speeches during the meeting with newly elected governors September 17, 2015 in Sochi, Russia. Putin said this week that it's impossible to defeat Islamic State group without support of the government of Syria and that Moscow has provided military assistance to President Bashar al-Assad's regime and will continue to do so. (Photo by Sasha Mordovets/Getty Images)
SOCHI, RUSSIA- NOVEMBER 26: Russian President Vladimir Putin greets Syrian Foreign Minister Walid Muallem in Bocharov Ruchey State Residence on November 26, 2014 in in Sochi, Russia. Foreign Minister of Syria is on a one-day visit to Russia. (Photo by Sasha Mordovets/Getty Images)
Head of the National Coordination Committee for Democratic Change, Hassan Abdel Azim (R) and other members of the Syrian tolerated opposition attend a meeting with Russia's Foreign Minister Sergei Lavrov in Moscow on August 31, 2015. AFP PHOTO / ALEXANDER NEMENOV (Photo credit should read ALEXANDER NEMENOV/AFP/Getty Images)
MOSCOW, RUSSIA - APRIL 29 : Russian Foreign Minister Sergei Lavrov (3-R), Russian President Vladimir Putin (3-R) and Presidential Aide Yury Ushakov (R) meet with members of the Elders group (L-R) former Algerian Foreign Minister and UN envoy to Syria Lakhdar Brahimi, Mexico's former President Ernesto Zedillo, former President of Finland and Nobel Peace Laureate Martti Ahtisaari, former UN Secretary-General, Nobel Peace Laureate and Chair of The Elders Kofi Annan, former president of the United States and Nobel Peace Laureate Jimmy Carter and former Prime Minister of Norway and Deputy Chair of The Elders Gro Harlem Brundtland at the Novo-Ogaryovo state residence outside Moscow, Russia, 29 April 2015. (Photo by Russia Kremlin Press-Office/Anadolu Agency/Getty Images)
Russia's President Vladimir Putin attends a ceremony of receiving foreign ambassadors' credentials in Aleksandrovsky (Alexander's) Hall in Grand Kremlin Palace in Moscow, on January 16, 2014. Russia launched today a fresh round of Syrian diplomacy on as it hosted the foreign ministers of Iran and Syria ahead of peace talks in Switzerland. AFP PHOTO / ALEXANDER NEMENOV (Photo credit should read ALEXANDER NEMENOV/AFP/Getty Images)
Russia's President Vladimir Putin (L) and Foreign Minister Sergei Lavrov attend a ceremony of receiving foreign ambassadors' credentials in in Grand Kremlin Palace in Moscow, on January 16, 2014. Russia launched today a fresh round of Syrian diplomacy on as it hosted the foreign ministers of Iran and Syria ahead of peace talks in Switzerland. AFP PHOTO / ALEXANDER NEMENOV (Photo credit should read ALEXANDER NEMENOV/AFP/Getty Images)
Russian President Vladimir Putin (R) welcomes Iran's Foreign Minister Mohammad Javad Zarif as they meet in Moscow's Kremlin, on January 16, 2014. Russia launched today a fresh round of Syrian diplomacy on as it hosted the foreign ministers of Iran and Syria ahead of peace talks in Switzerland. AFP PHOTO / POOL/ SERGEI KARPUKHIN (Photo credit should read SERGEI KARPUKHIN/AFP/Getty Images)
MOSCOW, RUSSIA - NOVEMBER 22: Turkish Prime Minister Tayyip Erdogan (R) and Russian President Vladimir Putin (L) arrive to the High-Level Russian-Turkish Cooperation Council meeting in the Konstantin Palace on November 22, 2013 in Saint Petersburg, Russia. Erdogan is on a two-day official visit to Saint Petersburg to attend the fourth session of the Russian-Turkish High-Level Cooperation Council with Russian President Vladimir Putin. The two leaders are expected to discuss economic development and cooperation, with the Syrian Crisis expected to be top the agenda. (Photo by Sasha Mordovets/Getty Images)
SOCHI, RUSSIA- NOVEMBER 26: Russian President Vladimir Putin greets Russian Foreign Minister Sergei Lavrov in Bocharov Ruchey State Residence on November 26, 2014 in in Sochi, Russia. Foreign Minister of Syria is on a one-day visit to Russia. (Photo by Sasha Mordovets/Getty Images)
Russian President Vladimir Putin (R) welcomes Syrian Foreign Minister Walid Muallem (L) during their meeting at the residence of Bocharov Ruchei in Sochi on November 26, 2014. President Vladimir Putin on Wednesday hosted Syrian Foreign Minister Walid Muallem in his first meeting with a top envoy from Damascus since the old Moscow ally plunged into civil war more than three years ago. AFP PHOTO / RIA NOVOSTI / KREMLIN POOL / ALEXEY DRUZHININ (Photo credit should read ALEXEY DRUZHININ/AFP/Getty Images)
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The Pentagon declined comment at a news briefing when asked about the Reuters report on Russian drones, saying it could not discuss intelligence matters. But it said the U.S. Department of Defense was "keenly aware" of what was happening on the ground in Syria.
The White House acknowledged that Moscow's intentions were unclear and that the prospect of deepening Russian military backing for Assad was troubling.
"We've made clear both in public and in private that doubling down on supporting Assad is a losing bet," said White House spokesman Josh Earnest.
One U.S. official, speaking on condition of anonymity, said the number of fixed-wing, piloted Russian aircraft stationed at the air base near Latakia, an Assad stronghold, had also grown dramatically in recent days.
That included Russia's positioning of a dozen "Fencer" advanced-attack aircraft and a dozen "Frogfoot" jets, used for close air support. Those were in addition to Russia's first deployment of fighter jets last week.
U.S. Secretary of State John Kerry said over the weekend the United States welcomed Russia's involvement in tackling Islamic State militants in Syria. But he said a worsening refugee crisis highlighted the need to find a compromise that could also lead to political change in the country.
Syria's civil war has killed an estimated 250,000 people, and many continue to flee their homes, with 4 million refugees and another 7.6 million displaced inside the country.
Russian President Vladimir Putin has pledged to continue military support for Assad, assistance that Russia says is in line with international law.
It was also unclear whether Moscow might eventually target opposition fighters that the United States supports in Syria, seeing them as equal threats to Assad as Islamic State fighters.
U.S. and Russian defense chiefs agreed on Friday to explore ways to avoid accidental interactions, also known as "deconfliction" in military parlance. But those discussions were described as only at their inception.
It was unclear whether the U.S.-Russian talks might gain added urgency, now that Moscow has started drone flights.
Russia's drone operations appeared to be staged out of the air base near Latakia, officials said.
Two U.S. officials, speaking on condition of anonymity, said it was not immediately clear how many Russian drone aircraft were operating or the scope of their missions.
At the State Department, spokesman John Kirby acknowledged concerns over the kind of Russian hardware being sent to Syria, saying it added to questions about whether Moscow's aim was mostly to battle Islamic State or to "prop up the Assad regime."
Meanwhile, Israel and Russia agreed to coordinate military actions over Syria in order to avoid accidentally trading fire. Israeli Prime Minister Benjamin Netanyahu said after talks in Moscow with Putin that they had "agreed on a mechanism to prevent such misunderstandings."