Russia has positioned about a half dozen tanks at an airfield at the center of a military buildup in Syria, two U.S. officials said on Monday, adding that the intentions of Moscow's latest deployment of heavy military equipment were unclear.
Moscow has come under increased international pressure in recent days to explain its moves in Syria, where the Kremlin has been supporting Syrian President Bashar al-Assad in a 4-1/2-year war.
The Pentagon declined to directly comment on the Reuters report, saying it could not discuss U.S. intelligence. But a U.S. Defense Department spokesman said recent actions by Moscow suggested plans to establish a forward air operating base.
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Russia positioning tanks at Syria airfield: 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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"We have seen movement of people and things that would indicate that they plan to use that base there, south of Latakia, as a forward air operating base," Pentagon spokesman Captain Jeff Davis told a news briefing.
One of the U.S. officials, who spoke to Reuters on condition of anonymity to discuss intelligence matters, said seven Russian T-90 tanks had been observed at the airfield near Latakia, an Assad stronghold.
The two U.S. officials said Russia had also stationed artillery that appeared to be arrayed defensively to protect Russian personnel stationed there.
Reuters has previously reported that Russia had deployed about 200 naval infantry soldiers to the airfield, as well as temporary housing units, a portable air traffic control station and components for an air defense system.
In a sign of the pace of Russia's buildup, Moscow has been sending about two military cargo flights a day to the airfield over the past week, U.S. officials say.
A diplomatic source, speaking to Reuters in the region on condition of anonymity, said the Russians were working to improve the airfield."There have been trucks going in and out. It appears the runway is not suited to some types of aircraft yet and they have been doing some improvements," the diplomat said.
Russia has said it will continue providing military supplies to Syria and that its assistance to the Syrian army is in line with international law.
The United States is using Syrian airspace to lead a campaign of air strikes against Islamic State. A greater Russian presence raises the prospect of the Cold War superpower foes encountering each other on the battlefield.
So far, Russia has not sent combat aircraft or helicopter gunships to the airfield, the Pentagon said.
Both Moscow and Washington say their enemy is Islamic State, whose Islamist fighters control large parts of Syria and Iraq. But Russia supports the government of Assad inSyria, while the United States says his presence makes the situation worse.
White House spokesman Josh Earnest told reporters on Air Force One that any Russian support to Assad would be "destabilizing and counterproductive." But the White House appeared to hold out hope for some degree of cooperation with Moscow, noting shared concerns about violent extremism.
State Department spokesman John Kirby said Russia's ultimate goal in Syria remained uncertain.
"Clearly, they are providing more assistance," Kirby said. "But the ultimate goal? The ultimate intent here? I think there's still a degree of opaqueness about that."
The Syrian civil war, in which about 250,000 people have died, has caused nearly half of Syria's prewar population of 23 million to flee, with many thousands attempting to reach Europe.
(Reporting by Phil Stewart,; additional reporting by Roberta Rampton aboard Air Force One, Lesley Wroughton and Arshad Mohammed in Washington and Sylvia Westall in Beirut; Editing by Susan Heavey, David Alexander and Jonathan Oatis)