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 inside the country since staging a rapid buildup at a Syrian air base.

The start 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 former Cold War foes have a common adversary in Islamic State militants in Syria. But Washington opposes Moscow's support for Syrian President Bashar Assad, seeing him as a driving force in the nation's 4½-year-old conflict.