Russia reportedly started flying drones over Syria for surveillance missions 22209151.

In Syria, both Russia and the U.S. are fighting the Islamic State, but Russia supports the government of President Bashar al-Assad while the U.S. says he makes the situation worse and is a driving force behind the four year civil war that has ravaged the country. Assad is currently engaged in conflicts with both ISIL and U.S. coalition-backed rebel groups.

U.S. defense officials say they are concerned Russian operations may also target U.S.-backed rebel groups should they threaten Assad.

The Pentagon declined comment on the matter, but said the Department of Defense was “keenly aware” of what was happening on the ground in Syria.

“We continue to believe that anything that’s done in support of the Assad regime, particularly militarily, is counter-productive and risks worsening an already bad situation,” Pentagon spokesman Capt. Jeff Davis told reporters Monday.

A U.S. official told CNN that 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.

Also on Monday, Israeli Prime Minister Binyamin Netanyahu met with Russian President Vladimir Putin to discuss security concerns along Israel’s northern border and how to avoid accidental clashes between Israeli and Russian forces. Netanyahu said they had “agreed on a mechanism to prevent such misunderstandings.”