By Michael Georgy

BAGHDAD, Nov 2 (Reuters) - 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 government said in the first official confirmation of the scale of the massacre.

The systematic killings, which one tribal leader said were continuing on Sunday, marked some of the worst bloodshed in Iraq since the Sunni militants swept through the north in June with the aim of establishing medieval caliphate there and in Syria.

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

"The number of people killed by Islamic State from Albu Nimr tribe is 322. The bodies of 50 women and children have also been discovered dumped in a well," the country's Human Rights Ministry said on Sunday.

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

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

Officials at a government security operations command center in Anbar and civilians reached by Reuters said they had not heard of or witnessed airstrikes.

STRATEGIC REGION

The fall of the village dampened the Shi'ite-led national government's hopes the Sunni tribesmen of Anbar -- who once helped U.S. Marines defeat al Qaeda -- would become a formidable force again and help the army take on Iraq's new, far more effective enemy.

U.S. airstrikes have helped Kurdish peshmerga fighters retake territory in the north that Islamic State had captured in its drive for an Islamic empire that redraws the map of the Middle East.

But the picture in Anbar is more precarious.

Islamic State already controls most of the vast desert province which includes towns in the Euphrates River valley dominated by Sunni tribes, running from the Syrian border to the western outskirts of Baghdad.

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

Ga'aud said 75 more members of his tribe were killed on Sunday under the same scenario -- they were hunted down while trying to escape from Islamic State, shot dead execution-style and dumped near the town of Haditha.

The Albu Nimr leader also said Islamic State killed 15 high school and college students in Zauiyat Albu Nimr and that, apart from an air drop, there had been no help from the U.S.-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 near Baghdad. (Editing by Philippa Fletcher)