Islamic State fighters holed up in one of the militant group’s last major strongholds in northern Syria have 48 hours to leave the city, according to an ultimatum issued by the local military council of the United States-supported Syria Democratic Forces.

The two-day window opened after the main Syrian exile opposition group called on the coalition to halt its bombing of militant positions near the city of Manbij after dozens of civilians were reported killed this week in airstrikes by the United States-led coalition.

The Syrian Observatory of Human Rights, based in Britain, has reported that more than 100 civilians have died in airstrikes in the area since late May, when militias supported by the United States and its coalition partners in Syria began an offensive to seize Manbij from the Islamic State, also known as ISIS and ISIL.

On Tuesday, the observatory reported that airstrikes north of Manbij killed at least 56 civilians, including 11 children. A day before, 21 people were killed in raids in the area also believed to have been conducted by coalition aircraft in Manbij.