The U.S. Defense Department says the U.S.-led coalition fighting the extremist group Islamic State (IS) will investigate reports that an air strike hit a school being used as a shelter in Syria, killing at least 33 people near the IS stronghold of Raqqa.
The Britain-based Syrian Observatory for Human Rights said it believed the air strike was carried out by aircraft of the U.S.-led coalition that is fighting IS in Syria and Iraq.
"At this time the coalition has no indication that an airstrike struck civilians near Raqqa as the Syrian Observatory for Human Rights claims," a statement from the U.S. Central Command said.
"However, since we have conducted several strikes near Raqqa, we will provide this information to our civilian casualty team for further investigation."
U.S.-backed Syrian Kurdish-Arab forces have been waging a campaign, with coalition-led air support, for months to retake Raqqa, the last major urban stronghold held by IS militants. IS has declared Raqqa its capital.