By Phil Stewart

WASHINGTON (Reuters) - 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.

"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.

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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 in Syria, 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)