The Russian Navy’s large landing ship Novocherkassk sails in the Bosphorus, on its way to the Mediterranean Sea, in Istanbul, Turkey.

Russia fired cruise missiles at Syrian targets this week without giving any advanced notice, U.S. Defense Secretary Ash Carter said on Thursday, describing a trend of unpredictable military action that has put the United States and its allies on edge.

“We’ve seen increasingly unprofessional behavior from Russian forces. They violated Turkish airspace … They shot cruise missiles from the Caspian sea without warning,” Carter said after NATO defense talks in Brussels.

Moscow said on Wednesday its warships fired a barrage of missiles at Syrian targets from the Caspian Sea, passing a distance of almost 1,500 km (900 miles) over Iran and Iraq to reach their targets.

A U.S.-led coalition is carrying out air strikes against Islamic State militants in both Iraq and Syria, a campaign that Washington has vowed will continue despite Russia’s escalating military activity in Syria.

The United States has acknowledged that U.S.-led coalition aircraft bombing Islamist militants in Syria were re-routed at least once to avoid a close encounter with Russian planes. Carter said Russian aircraft came within a few miles of a U.S. drone.

The United States, which opposes Russia’s backing of Syrian President Bashar al-Assad, has ruled out any military cooperation with Russia in Syria.

But the Pentagon is seeking to work out an agreement with Moscow on operating procedures to ensure pilot safety.