Survivors tell the same story: they were taken from their homes by men in uniform; heads down and linked together, then led in small groups to a field, made to kneel, and selected to be shot one by one.

Accounts by five witnesses interviewed separately by Reuters provide a picture of alleged executions in the eastern village of Barwanah on Monday, which residents and provincial officials say left at least 72 unarmed Iraqis dead.

The witnesses identified the killers as a collection of Shi'ite militias and security force elements.

Iraqi security and government officials have disputed the accounts, with some saying radical jihadists from Islamic State could have perpetrated the killings.

The government said on Wednesday it was opening a probe into the killings.

"The prime minister has ordered an urgent investigation and we are awaiting the results," said Prime Minister Haider al-Abadi's spokesman, Rafid Jaboori. "I don't want to come to any conclusions now. When the results of this investigation come out, we will have a full picture."

Iraq's Shi'ite-led government, backed by US-led air strikes, has been trying to push back Islamic State since it swept through northern Iraq in June.

Monday's alleged massacre followed a three-day offensive in which Shi'ite militias and Iraqi security forces captured two dozen villages from Islamic State near the town of Muqdadiya in Diyala province.

Since September, hundreds of civilians have fled to Barwanah's relative safety from fighting in Sinsil, about 5 km (3 miles) to the southwest, and other nearby villages.

TAKEN OUT AND SHOT

Abu Omar, a businessman displaced from Sinsil, was at home in Barwanah on Monday around 3:30pm when about 10 Humvees arrived carrying a few dozen men.

Black and brown uniforms suggested some were affiliated with Shi'ite militias and government security forces; others appeared to be civilians.

They dragged residents up to age 70 from their homes, beating and cursing them with sectarian slurs, Abu Omar told Reuters by phone.

He said the fighters took the men's mobiles and ID cards, then bound their hands, tying Abu Omar to his 12-year-old mentally ill son with rope. They did the same with his two older sons and three brothers.

The men were led a few hundred yards to a field where Abu Omar said more than a hundred others had been gathered.

For about two hours, they were forced to kneel and stare at the ground as the fighters selected their targets and led them to a spot behind a mud wall.

"They took them behind the wall. Less than a minute, then a gunshot," said Abu Omar. "All we could hear was the gunshots. We couldn't see."

Survivors say victims were taken also to alleyways, houses, behind a mosque, or an area used to collect garbage, and then shot.

Abu Maz'el, 25, a farmer from Sinsil who was displaced to Barwanah five months ago, gave Reuters nearly identical testimony.

He said some of the fighters wore green headbands emblazoned with the name Hussein, a defining figure in Shi'ite history.

They took him and his cousin from their home to the field, walking single file, heads down, with their hands on the other men's shoulders.

Kneeling beside his 35-year-old cousin, Abu Maz'el heard others beg for their lives as the gunmen dragged them off and shot them.

"My cousin raised his head, so someone slapped him," he said. "Five minutes later, they came and took him away and executed him."

"FALLING LIKE DOMINOS"

Diyala has been plagued by sectarian violence with Islamic State and Shi'ite militias fighting for control of the strategic region northeast of Baghdad.

Sunni militants carry out frequent suicide bombings and assassinations. In turn, Shi'ite militias have been accused of carrying out killings of Sunnis, including two other mass atrocities in Diyala in the last year.

Monday's alleged massacre happened in the presence of Iraqi security forces, compounding Sunni doubts about Baghdad's control over the militias, which took the lead in battling Islamic State after the Iraqi army nearly collapsed last summer.

Abdullah al-Jubouri, a 23-year-old college graduate who fled to Barwanah from Sinsil a month ago, said the army let him go when they came to his house on Monday. Other witnesses told Reuters that soldiers stood by helplessly, some crying, as the militias executed civilians.

Jubouri told Reuters he fled when he saw Humvees entering Barwanah and hid in a pile of garbage. He watched as a group of soldiers and militiamen near the school fired at a line of 13 men, some with their hands bound.

"I saw them falling like domino pieces," he said.

SURROUNDED

Jubouri said he heard shots and screams until about 7 p.m., when the vehicles left. He discovered a neighbour and his two sons among the bodies by the school.

Women and children emerged to cover the men's bodies. Some spent the night in the streets mourning the dead.

Jubouri said he found the body of another neighbour outside his house with bullet wounds to his head and chest. He saw bodies with similar wounds in the field and in five separate streets throughout the village.

Abu Omar, the businessman, returned home after the fighters withdrew, and was reunited with his sons. He later found six brothers also from Sinsil had been killed, one at his home and the others behind the mud wall in the field.

A cosmetics salesman and four teachers were killed in the field, along with three other brothers and their cousin, Abu Omar said.

Haqqi al-Jobouri, a Sunni member of the Diyala provincial council, told Reuters at least 72 men were killed in Barwanah on Monday. He said 35 others were missing and suspected detained by the militias.

Sajid al-Anbuki, a Shi'ite member of the same body, urged restraint in drawing conclusions ahead of the government's investigation.

"If it revealed that those men executed were terrorists, then we don't have any problem because in this case they got what they deserve," he told Reuters.

"If the findings prove they were civilians, then justice should be done and those who did it must be arrested."

In the meantime, remaining residents of Barwanah fear further violence. They told Reuters the same militias and security forces encircled the village late on Monday, preventing anyone from leaving.

"We have been surrounded for days," said Abu Ahmed, 27, another survivor from Sinsil. "We have no food. We have nothing."