BEIRUT (Reuters) - Air strikes killed 23 civilians, including eight children, on Thursday in countryside around the northern Syrian city of Raqqa, the Islamic State’s base in Syria, a war monitoring group said.

The warplanes were believed to belong to the U.S.-led coalition against the Islamic State militants, the Britain-based Syrian Observatory for Human Rights said.

The jets struck the village of al-Matab southeast of Raqqa, near the Euphrates River, the Observatory said, adding that several air raids had also pounded areas east of the city.

An alliance of Syrian militias has been waging an offensive on Raqqa with air strikes and special ground forces from the U.S.-led coalition.

The U.S.-backed Syrian Democratic Forces cut the last main road out of Raqqa this week, severing the highway between the city and the militant group’s stronghold of Deir al-Zor.

“We are going to investigate the civilian casualty incident... we did conduct strikes in that area. We can’t verify whether or not it was one of ours,” said the U.S.-led coalition spokesman U.S. Air Force Colonel John Dorrian.

“We do everything that we can to avoid civilian casualty incidents,” he said by phone.

The military’s estimates of civilians killed by coalition air strikes are generally far lower than those of monitoring groups.