Su-27 jets of aerobatics team Russkiye Vityasy, or Russian Knights, perform during the MAKS-2015 International Aviation and Space Show in Zhukovsky, outside Moscow, Russia.

Russia’s latest movements at an airfield in Syria suggest plans to establish a forward air operating base, the Pentagon said on Monday, generally describing Russian movement of “people and things” in Syria but declining to offer specifics on U.S. intelligence.

“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,” said Pentagon spokesman Captain Jeff Davis.

Russian tanks in Syria

Meanwhile, Russia has also positioned about a half dozen tanks at a Syrian airfield at the center of a military buildup, two U.S. officials said on Tuesday, adding the intentions of Moscow’s latest deployment of heavy military equipment to Syria was unclear.

One of the U.S. officials said seven Russian T-90 tanks were observed at the airfield near Latakia, a stronghold of Syrian President Bashar al-Assad.

The two U.S. officials said Russia had also positioned artillery, which they said appeared to be arrayed defensively to protect Russian personnel stationed there.

Reuters has previously reported that Russia had deployed about 200 naval infantry forces to the airfield. Moscow has also been sending about two cargo flights a day to the airfield over the past week, U.S. officials say.

Moscow has come under increased international pressure in recent days to explain what Washington and Gulf states say is a significant Russian military build-up in Syria, where the Kremlin has been supporting Assad in a four-and-a-half-year war.

Russia has said it will continue with military supplies to Syria and that its assistance to the Syrian army is in line with international law.

The United States is using Syrian air space to lead a campaign of air strikes against Islamic State of Iraq and Syria (ISIS), and a greater Russian presence raises the prospect of the Cold War superpower foes encountering each other on the battlefield.

Both Moscow and Washington say their enemy is ISIS, 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.

The Syrian civil war, in which about 250,000 people have died, has caused nearly half Syria’s pre-war 23 million strong population to flee, with many thousands attempting to reach Europe.