A senior U.S. military official says the United States is “certain” that it was a Russian airstrike that killed three Syrian soldiers and wounded 13 others.

The U.S. has said it was not a coalition strike, but Syria’s government accused the U.S.-led alliance of bombing the army camp and hitting troops loyal to President Bashar Assad.

The city of Der el-Zour, where the strikes are said to have been carried out on Sunday, is mainly held by the Islamic State group, but the Syrian government maintains a presence in some parts of it.

The official was not authorized to discuss the matter publicly, so spoke on condition of anonymity.

(AP)