Islamic State militants have killed 322 members of an Iraqi tribe in western Anbar province, including dozens of women and children whose bodies were dumped in a well.

The details came in a government statement that was the first official confirmation of the scale of the massacre.

The systematic killings, which one tribal leader said were continuing yesterday, marked some of the worst bloodshed in Iraq since the Sunni militants swept through the north in June.

Islamic State (IS) aims to establish a medieval-style caliphate there and in neighbouring Syria.

The Albu Nimr tribe, also Sunni, had put up fierce resistance against IS for weeks but finally ran low on ammunition, food and fuel last week as IS fighters closed in on their village, Zauiyat Albu Nimr.

"The number of people killed by Islamic State from Albu Nimr tribe is 322," the country's Human Rights Ministry said yesterday.

"The bodies of 50 women and children have also been discovered dumped in a well."

One of the leaders of the tribe, Sheikh Naeem al Ga'oud, said he had repeatedly asked the central government and army to provide his men with arms but no action was taken.

State television said yesterday that Prime Minister Haider al Abadi had ordered airstrikes on IS targets around the town of Hit in response to the killings.

However, officials at a ­government ­security operations command centre in Anbar said they had not heard of or witnessed any airstrikes.

The fall of the village dampened the Shi'ite-led national government's hopes the Sunni tribesmen of Anbar would become a formidable force again.

The tribesmen once helped US Marines defeat al Qaeda and it had been hoped they would help the army take on Iraq's new, far more effective enemy.

US airstrikes have helped ­Kurdish peshmerga fighters retake territory in the north that IS had captured in its drive for an Islamic empire that would redraw the map of the Middle East.

But the picture in Anbar is more precarious.

IS controls most of the vast desert province - which includes towns in the Euphrates River valley dominated by Sunni tribes -running all the way from the Syrian border to the western outskirts of Baghdad.

If the province falls, it could give IS a better chance to make good on its threat to march on the capital.

The massacre was reportedly continuing yesterday, with Shiekh Ga'oud saying 75 more members of his tribe were killed. He said they were hunted down while trying to escape from IS and were shot dead in the manner of an execution and dumped near the town of Haditha.

The Albu Nimr leader also said IS killed 15 high school and college students in Zauiyat Albu Nimr.

He said that apart from an air drop there had been no help from the US-led air campaign.

Security and government ­officials could not be immediately reached to confirm the latest killings.

In Anbar, the militants are now 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 that are close to Baghdad.