Baghdad: 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 caliphate there and in Syria.

Local tribal fighters dig in against Islamic State militants in the town of Amriyat al-Falluja in Anbar province. Credit:Reuters

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.