At least 220 members of a tribe that opposed the Islamic State were found this week to have been executed, according to a Reuters report.
The news agency said the bodies of the men aged 18 to 55 were found in two mass graves in Anbar province on Thursday, one of the graves being close to the city of Ramadi and containing 150 bodies, while the other was uncovered near to the town of Hit holding the bodies of 70 men.
The victims were among 300 members of the Sunni Muslim Albu Nimr tribe that were taken captive earlier this week by IS militants.
Witnesses told Reuters that the men had been shot at close range and militants were using them as an example to deter other Iraqis from opposing the Islamic State.
"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.
Witnesses said most of the men were police or belonged to the Sahwa (Awakening) militia group.
Reuters said that with government and Kurdish fighters recapturing territory in the north of Iraq, the Islamic State has started to push hard into Anbar province as it keeps its sights on Baghdad.
It was reported this week that IS militants have taken hold of oilfields and refineries in northern Iraq and are using money raised through smuggling to finance their operations.
The US said it is working with the regional government in Arbil to break the smuggling networks.
Over in Syria, a fierce battle continues to be waged for the city of Kobani, close to the Turkish border. A report in The Economist today said it was still not possible to say which side will prevail in the city, with Kurdish defenders appearing to be outnumbered.
"IS is up against a foe with nowhere to retreat, an improving supply supply of arms and continued air support. Kobani may not be Stalingrad, but if it holds out, the psychological damage to IS will be real," it said.