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 were established with the encouragement of the United States to fight al Qaeda during the U.S. "surge" offensive of 2006-2007.
Washington, which no longer has ground forces in Iraq but is providing air support, hopes the government can rebuild the shaky alliance with Sunni tribes, particularly in Anbar which is now mostly under the control of Islamic State, a group that follows an ultra-hardline version of Sunni Islam.
But Sunni tribal leaders complain that Shi'ite Prime Minister Haider al-Abadi has failed to deliver on promises of weapons to counter Islamic State's machineguns, sniper rifles, rocket-propelled grenades and tanks.
Sheikh Naeem al-Ga'oud, one of the leaders of the Albu Nimir tribe, said: "The Americans are all talk and no action."
Islamic State was on the march in Anbar this year even before it seized much of northern Iraq in June. As the government and fighters from the autonomous Kurdish region have begun to recapture territory in the north, Islamic State has pressed its advances in Anbar, coming ever closer to Baghdad.
In the north, government forces said they were closing in on the city of Baiji from two sides on Thursday in an attempt to break Islamic State's siege of Iraq's biggest oil refinery.
A member of the Iraqi security forces said they might enter the city in the next few hours but he acknowledged that roadside bombs and landmines were slowing the advance.
"Now we are close to the checkpoint of southern Baiji, which means less than 500 metres from the town," he said, requesting anonymity. "We haven't seen strong resistance by them (Islamic State) but we are stopping every kilometre to defuse landmines."
His account could not be independently confirmed.
Islamic State fighters seized Baiji and surrounded the sprawling refinery in June during a lightning offensive through northern Iraq. The group also controls a swathe of territory in neighbouring Syria and has proclaimed a caliphate straddling both countries.
Iraqi Kurdish peshmerga fighters entered the Syrian town of Kobani on Thursday to help efforts to push back Islamic State militants who have besieged the town for the last 40 days.
Islamic State, an al Qaeda offshoot, has beheaded or shot dead anyone it captures who opposes its ideology. Its gunmen systematically executed about 600 inmates from Badoush Prison near the city of Mosul in June, Human Rights Watch said on Thursday.
Citing the accounts of 15 survivors, it said the group singled out Shi'ite prisoners, forced them to kneel along the edge of a nearby ravine and shot them with assault rifles and automatic weapons.