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 US-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 US-led coalition.
The US-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.
The US military has said it makes "extraordinary efforts" to avoid civilian deaths in its bombing campaign against Islamic State in Syria and Iraq.
The military's estimates of civilians killed by coalition air strikes are generally far lower than those of monitoring groups.
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