Syrian President Bashar Assad's government accused the U.S.-led coalition of bombing an army camp and killing three soldiers. A spokesman for the coalition denied the claim on Monday.

The Syrian government said three soldiers were killed and 13 were wounded in the attack, which it called an act of aggression. The jets fired nine missiles at the camp in Deir el-Zour province on Sunday night, Syria's Foreign Ministry said in a statement flashed on state television.

It is the first publicly declared incident of its type since the United States and allied countries began bombing the ISIS in Syria more than a year ago.

Syria's foreign ministry sent letters to the U.N. secretary general and to the head of the U.N. Security Council condemning the "flagrant aggression ... which goes firmly against the aims of the U.N. charter," state news agency SANA said.

It urged the United Nations to take "immediate action and take the necessary measures to prevent a repeat" of the incident.

But Col. Steven Warren, a spokesman for the U.S.-led coalition, denied the government's claim. "We did not hit them," he added.

Russia is waging its own aerial campaign in support of Assad, which has hit some ISIS targets, but the United States and its allies say most of the Russian raids have hit other foreign-backed rebel groups.