Since mid-June, around 13,000 Turkmens, including 10,000 women and children, have been besieged by ISIS in Salah al-Din, a governorate northwest of Baghdad, “and there are fears of a possible imminent massacre,” she said.

In a statement that puts on official record events reported in the news media, Ms. Pillay said ISIS forces had killed hundreds of people mainly from the Yazidi minority and abducted 2,500 others in Nineveh governorate at the beginning of August. Hundreds more from the Sinjar area were killed and abducted later in the month, she said. Men who refused to convert to Islam were executed, and women and children were handed over to ISIS fighters as slaves or were threatened with being sold, Ms. Pillay said.

The United Nations has verified reports of a massacre at Badoush Prison in Mosul in early June, Ms. Pillay said, when ISIS gunmen took as many as 1,500 prisoners to an uninhabited area nearby, separated the Sunni men, and forced the others to kneel in four rows before opening fire on them. Up to 670 prisoners were reported killed, Ms. Pillay said.