The Islamic State in Iraq militant group killed at least 220 Iraqis who opposed them in retaliation for taking over territory west of Baghdad, security sources and witnesses to the massacre said.
Two mass graves were found on Thursday and contain around 300 members of the Sunni Muslim Albu Nimr tribe, which IS occupied last week, Radio Free Europe reports.
The prisoners, men between the ages of 18 and 55, were killed at close range, witnesses said, Reuters reports.
The bodies of more than 70 men were dumped near the city of Hit in the heart of the Sunni province of Anbar, and according to witnesses, most of the victims were members of the police or the Sahva (Awakening) militia that fought against the Islamic State.
IS ordered the men from the tribe to leave their villages and head for the town of Hit, 130 kilometers from Baghdad, promising them "safe passage".
Then they surrounded them and killed them.
Another mass grave near the city of Ramadi, also in Anbar province, contains 150 bodies of members of the same tribe, security officials said.
The Sahva militia was established to help fight Al Qaeda during the US offensive in 2006-2007.
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