The U.S. military says Sept. 17 airstrikes in eastern Syria that reportedly killed dozens did not violate the international law of armed conflict.

An Air Force general who led a coalition investigation of the attacks told reporters Tuesday at the Pentagon that the targets were believed, incorrectly, to be Islamic State fighters. The investigation determined they actually were fighters affiliated with the Syrian armed forces.

The U.S., Australia, Denmark and Britain participated in the attacks.

Syria denounced the attack and said it helped the Islamic State group.

Brig. Gen. Richard Coe said that because the mistake was not deliberate or a result of negligence, the attacks were not illegal.

Coe said investigators confirmed 15 deaths but believe there were more. He said outside observers say more than 80 people died.