Russia has started flying drone aircraft on surveillance missions in Syria, two 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 U.S. officials, who spoke to Reuters on condition of anonymity, could not immediately say how many Russian drones were involved in the surveillance missions or the scope of the flights.

The Pentagon declined to comment. The start of Russian drone flights underscored the risks of U.S.-led coalition aircraft and Russian flights operating in Syria’s limited airspace. U.S. and Russian defense chiefs agreed on Friday to explore ways to avoid accidental interactions, also known as “deconfliction” in military parlance.The discussions may gain added urgency, now that Moscow has started drone flights. FULL REPORT