At least 56 civilians were reported killed on Tuesday in airstrikes north of the besieged Islamic State-held city of Manbij in northern Syria, and residents said they believed the attack was carried out by aircraft from the United States-led coalition, a monitoring group said.

The Britain-based Syrian Observatory for Human Rights said that the dead included 11 children, and that dozens more people were wounded.

The so-called Syria Democratic Forces, an alliance of Kurdish and Arab fighters backed by the coalition, started an offensive at the end of May to seize Manbij. The area is among the last major territory held by Islamic State militants on Syria’s frontier with Turkey.

Supported by American-led coalition airstrikes, the Kurdish forces have surrounded the city. But Islamic State attacks still occur in some areas of the surrounding countryside.