BAGHDAD — “Islamic State” militants executed at least 220 Iraqis in retaliation against a tribe’s opposition to their takeover of territory west of Baghdad, security sources and witnesses said.

Two mass graves were discovered on Thursday containing some of the 300 members of the Sunni, Muslim Albu Nimr tribe that “Islamic State” had seized this week. The captives, men aged between 18 and 55, had been shot at close range, witnesses said.

The bodies of more than 70 Albu Nimr men were dumped near the town of Hit in the Sunni heartland Anbar province, according to witnesses who said most of the victims were members of the police or an anti-“Islamic State” militia called Sahwa (Awakening).

“Early this morning we found those corpses and we were told by some “Islamic State” militants that ‘those people are from Sahwa, who fought your brothers the “Islamic State,”  and this is the punishment of anybody fighting ‘Islamic State’,” a witness said.

The insurgents had ordered men from the tribe to leave their villages and go to Hit, 130 km (80 miles) west of Baghdad, promising them “safe passage”, tribal leaders said. They were then seized and shot.

A mass grave near the city of Ramadi, also in Anbar province, contained 150 members of the same tribe, security officials said.

The Awakening militia was established with the encouragement of the United States to fight al Qaeda during the U.S. “surge” offensive of 2006-2007.