Syria’s military says the U.S.-led coalition struck its base in the eastern province of Deir el-Zour, allowing the Islamic State group to advance in the fiercely contested area.

There was no immediate comment from Washington. If confirmed, it would be the first time the coalition has targeted Syrian government forces.

In a statement Saturday, the Syrian military says the airstrikes caused casualties and damage to equipment, and enabled an IS advance on the hill overlooking the air base.

The statement calls it a “serious and blatant attack on Syria and its military” and “firm proof of the U.S. support of Daesh and other terrorist groups.” Daesh is an Arabic acronym for IS.

Syrian President Bashar Assad’s government views all those fighting against it as “terrorists,” and has long accused the U.S. and other rebel supporters of backing extremists.