United Nations human rights chief Navi Pillay on Monday condemned "appalling, widespread" crimes being committed by Islamic State forces in Iraq, including killings, slavery, sexual crimes and targeting people on ethnic or religious grounds.

Up to 670 prisoners from Badush prison in the city of Mosul were killed by Islamic State on June 10 after being taken by truck to a vacant area and screened for non-Sunnis, she said in a statement quoting survivors and witnesses to the "massacre" as telling U.N. human rights investigators.

"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 in a statement issued in Geneva.