Two Russian military ships believed to be carrying equipment and supplies are headed to Syria, defense officials told NBC News on Wednesday.
The Russians' intentions remain unclear, officials said. Moscow has previously sent humanitarian assistance to the Syrians, but another U.S. official said analysis suggests Moscow is setting up a site for a squadron to go in, presumably to help the regime of President Bashar Assad with airstrikes.
Al Qaeda-linked rebels have recently seized a number of cities in a four-year civil war that has killed at least 250,000 people. They forced government troops out of Abu al-Duhur, a major air base in the northwestern province of Idlib, after a two-year siege, state TV said Wednesday.
Russia has refused to detail the scope of its military presence in Syria. The U.S. says it is believed to have fewer than 100 military personnel on the ground, setting up mobile housing units and a portable air traffic control tower at an airstrip outside Latakia.
But "we have been informed that the Russians are entering into active intervention," Amos Gilad, a senior adviser to Israeli Defense Minister Moshe Yaalon, said Tuesday at a security conference in Israel.
Syria hasn't formally responded to the reports.
The State Department said Secretary of State John Kerry spoke with Russian Foreign Minister Sergei Lavrov over the weekend to raise concerns about the reports of "an imminent enhanced Russian military buildup" in Syria.
"The secretary made clear that if such reports were accurate, these actions could further escalate the conflict, lead to greater loss of innocent life, increase refugee flows and risk confrontation with the anti-(ISIS) coalition operating in Syria," the State Department said.