GENEVA (AP) — Islamic State fighters reportedly killed up to 670 prisoners in Mosul and committed other horrific abuses in Iraq that amount to crimes against humanity, UN High Commissioner for Human Rights Navi Pillay said Monday.

The UN’s top human rights official said “grave, horrific human rights violations are being committed daily” by the Islamic State group and other fighters allied with it in an aggressive push to gain a firm grip on the northern and eastern provinces.

The group’s violations as it expands the boundaries of its self-proclaimed caliphate along the Syria-Iraq border include targeted killings, forced conversions, abductions, trafficking, slavery, sexual abuse, destruction of places of religious and cultural significance, and besieging entire communities for ethnic, religious or sectarian reasons, Pillay said.

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“They are systematically targeting men, women and children based on their ethnic, religious or sectarian affiliation and are ruthlessly carrying out widespread ethnic and religious cleansing in the areas under their control,” she said. “Such persecution would amount to crimes against humanity.”

Pillay cited the killing of hundreds of Yazidis in Nineveh and up to 2,500 kidnapped at the beginning of August, and the killing and abduction of hundreds of Yazidis in Cotcho village in Southern Sinjar on August. 15. She also pointed to at least 13,000 Shia Turkmen in the town of Amirli, including 10,000 women and children, who have been besieged since June 15.

The UN mission in Iraq has also verified reports of a massacre of prisoners and detainees from Mosul’s Badoush Prison on June 10, she said. Interviews with 20 survivors and 16 witnesses described Islamic State gunmen loading between 1,000 and 1,500 prisoners onto trucks and driving them to a nearby uninhabited area. Armed men told the Sunnis to separate themselves from the others.

According to the accounts, gunmen then yelled insults at the remaining prisoners, lined them up in four rows, ordered them to kneel and opened fire, reportedly killing up to 670 prisoners.

“Such cold-blooded, systematic and intentional killings of civilians, after singling them out for their religious affiliation may amount to war crimes and crimes against humanity,” Pillay said.