Russia has started flying drone aircraft on surveillance missions in Syria, U.S. officials said on Monday, in what appeared to be Moscow"s first military air operations there since staging a rapid buildup at a Syrian air base.

The beginning of Russian drone flights underscored the risks of U.S.-led coalition planes and Russian aircraft operating within Syria"s limited airspace, without agreeing on coordination or objectives in Syria"s civil war.

The Pentagon declined comment at a news briefing when asked about the Reuters report on Russian drones, saying it could not discuss intelligence matters. But it said the U.S. Department of Defense was "keenly aware" of what was happening on the ground in Syria.

The White House acknowledged that Moscow"s intentions were unclear and that the prospect of deepening Russian military backing for Assad was troubling.

"We"ve made clear both in public and in private that doubling down on supporting Assad is a losing bet," said White House spokesman Josh Earnest.

At the State Department, spokesman John Kirby acknowledged concerns over the kind of Russian hardware being sent to Syria, saying it added to questions about whether Moscow"s aim was mostly to battle Islamic State or to "prop up the Assad regime."