An international rights group said Islamic State militants carried out a mass killing of hundreds of Iraqi prison inmates when they seized the country's second-largest city of Mosul in June, the Associated Press reports.

Human Rights Watch said Thursday, Oct 30, that some 600 male Shiite inmates from Badoosh prison outside Mosul were forced to kneel along the edge of a nearby ravine and shot with automatic weapons.

The New York-based watchdog said in a statement that the Shiite prisoners were separated from several hundred Sunnis and a small number of Christians who were later set free. The statement was based on interviews with survivors.

Also in June, the Islamic State group claimed it had "executed" about 1,700 soldiers and military personnel captured from Camp Speicher outside Tikrit city.