Islamic State, also known as ISIS or ISIL, executed 85 more members of the Albu Nimr tribe in Iraq in a mass killing campaign launched last week to break their resistance to the group's territorial advances, a tribal leader and security official said on Saturday.
Sheikh Naeem al-Ga'oud, one of the tribe's chiefs, told Reuters that Islamic State had killed 50 members of Albu Nimr who were fleeing the al Qaeda offshoot on Friday.
In a separate incident, a security official said 35 bodies were found in a mass grave.
The sustained bloodshed appears to demonstrate Islamic State's resilience to US airstrikes against militant targets in parts of Iraq and Syria it controls. Ga'oud said he had repeatedly asked the Shi'ite-led central government for weapons but his pleas had been ignored.
Islamic State has been killing at will, with no signs that the Shi'ite-led government will send armed forces to the rescue of Albu Nimr or other tribes under threat anytime soon.
Members of the Albu Nimr tribe had held out for weeks under siege by Islamic State fighters in Anbar Province to the west of Baghdad, but finally ran low on ammunition, fuel and food.
Hundreds of tribal fighters withdrew and members of the tribe fled their main village Zauiyat albu Nimr.
Islamic State rounded up many of them, shot them at close range and dumped them in mass graves. Over 300 people have been executed since the killing began in the middle of last week, Ga'oud and security officials.
Security officials and witnesses have confirmed that bodies of more than 200 people were found in mass graves on Tuesday and Wednesday, having been shot at close range.
Islamic State squeezes vast Anbar province
The vast desert province of Anbar includes towns in the Euphrates River valley dominated by Sunni tribes, running from the Syrian border to the western outskirts of Baghdad.
It was the main battleground between US Marines and al Qaeda during the "surge" campaign in 2006-2007, the bloodiest phase of the US war in Iraq, when American troops enlisted the tribes to help them defeat al Qaeda fighters.
Members of the Albu Nimir tribe helped the Americans defeat al Qaeda in its stronghold of Anbar.
Now they are on the defensive as Islamic State militants who possess superior weapons exact revenge, including executions of tribesmen in public squares, for weeks of defiance.
In Anbar, the militants are encircling a large air base and the vital Haditha dam on the Euphrates. Fighters control towns from the Syrian border to parts of provincial capital Ramadi and into the lush irrigated areas near Baghdad
Ga'oud said the 50 tribe members were killed near Tharthar Lake near a desert area.
They had been wandering by foot when they were intercepted by the Sunni militants.
He said one managed to escape the carnage and get word to tribal leaders.
"Forty of the dead were men. Six women and four children were killed while trying to protect their husbands and fathers," said Ga'oud.
"We could not send people to retrieve the bodies because the area is still controlled by Islamic State."
His account was confirmed by Faleh al-Essawi, the chief of the security committee of the Anbar Provincial Council.
In another incident 35 corpses were found on the outskirts of Ramadi. Security officials said they were dumped in a mass grave. But an eyewitness said the militants left the bodies on the ground near farmland.
"The corpses were dumped two days ago. They were handcuffed and blindfolded. Some were wearing tracksuits and others were wearing dish-dash robes," he told Reuters.
Shi'ite Prime Minister Haider al-Abadi wants Sunni tribal leaders to support Iraq's army against Islamic State, which has threatened to march on Baghdad as part of an ambition to redraw the map of the Middle East.
But mistrust had undermined efforts to revive an alliance, with tribal figures accusing Abadi of breaking promises to provide them with weapons.
Iraq's US-trained military collapsed in the face of an Islamic State sweep through the north in June. The Sunni militants seized a range of weapons from defeated troops, including machineguns and tanks.
Iraq's most senior Shi'ite cleric, Grand Ayatollah Ali al-Sistani, called on the government on Friday to rush to the aid of Sunni tribal leaders.