Two U.S. officials said on Monday, adding that the intentions of Moscow's latest deployment of heavy military equipment were unclear, Censor.NET reports citing Reuters.
The Pentagon declined to directly comment on the Reuters report, saying it could not discuss the U.S. intelligence. But a spokesman said recent actions by Moscow suggested plans to establish a forward air operating base.
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.
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 in Syria, while the United States says his presence makes the situation worse.