The Syrian Observatory for Human Rights said on Tuesday dozens of people were killed in an air strike believed to have been carried out by the U.S.-led coalition on an Islamic State prison in the eastern Syrian town of al-Mayadeen.

The coalition said it had carried out strikes on known IS targets in the town on Sunday and Monday — the day the Observatory said the prison was hit, killing 57 people.

The coalition said the mission had been “meticulously planned” to reduce the risk of possible harm to non-combatants. It added it would assess the Observatory’s allegation.

The Islamic State is believed to have moved most of its leadership to al-Mayadeen in Syria’s Euphrates Valley, southeast of the group’s besieged capital Raqqa, according to U.S. intelligence officials.

Among operations moved to al-Mayadeen, about 80 km west of the Iraqi border, are its online propaganda operation and its limited command and control of attacks in Europe and elsewhere, they say. The Observatory said the air strike took place on Monday at dawn, hitting a building in the town of al-Mayadeen that was being used as a prison.

The U.S. is supporting an alliance of Kurdish and Arab fighters in their assault on IS in Raqqa.