Story highlights The introduction of drones follows a buildup of Russian military equipment and personnel into Syria.

Defense Secretary Ashton Carter spoke with Russian Defense Minister Sergei Shoigu Friday on the escalating Russian posture.

Washington (CNN) Russia has started to fly unmanned aircraft over Syria in what appears to be surveillance operations over the country, two U.S. officials told CNN.

The officials were not able to confirm whether the drones were armed, or how many missions have been flown to date. The development was first reported by Reuters.

The introduction of drones follows a buildup of Russian military equipment and personnel into Syria.

To date, Russia has over 25 fighter and attack aircraft, 15 helicopters, nine tanks, three surface-to-air missile systems and at least 500 personnel on the ground in Syria, a U.S. official told CNN.

Defense Secretary Ashton Carter spoke with Russian Defense Minister Sergei Shoigu on Friday regarding the escalating Russian posture.

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