The latest attack, which brings to at least 322 the number of Al-Bu Nimr members killed by the Islamic State in just two weeks, took place in the village of Ras al-Maa in the north of Ramadi, the Iraqi Human Rights Ministry said on Sunday.

"The bodies of 50 women and children have been discovered dumped in a well," the ministry told Reuters in the first official confirmation of the attack.

Local media said Prime Minister Haider al-Abadi had ordered airstrikes on Islamic State targets around the town of Hit, the home of the Al-Bu Nimr tribe, in response to the killings.

The latest victims – 40 men, six women and four children – were lined up in public and shot, senior tribal member Sheikh Naim al-Gaoud told Associated Press, while a further 17 people were kidnapped.

Another 50 members of the tribe were killed on Friday and at least 47 were killed on Thursday in what commentators are describing as a stark warning against those who dare to resist the Islamic State's push to expand its control over parts of Iraq and Syria.

Much of the western province of Anbar is under the control of the Islamic State, whose militants quickly moved from their takeover of Iraq's second city of Mosul in June to consolidate their hold over Tikrit in the north, before working their way south towards the capital Baghdad.

The United Nations Security Council condemned the "kidnapping and murder of scores of Sunni tribesmen".

"Many of these tribesmen had been combating terrorism, together with the Government of Iraq," the Security Council said on Sunday.

"This crime, like the Camp Speicher massacre in Tikrit, once again demonstrated ISIL's brutality, clearly showing that terrorist groups in Iraq target all segments and sects of the Iraqi population."

October was a bloody month in Iraq. At least 1273 people were killed and a further 2010 injured in what the UN described as "acts of terrorism and violence", warning that these figures must be considered the "absolute minimum" toll.

Sunni tribes, who united to repel al-Qaeda from Iraq in 2007-2008 only to face repression and alienation from the Shiite-led government of former prime minister Nouri al-Malaki, are seen as a key plank in Iraq's defence against the Islamic State.

While Mr Malaki was in power they were reluctant to join the fight against IS until their demands for better treatment by the government, an end to arbitrary arrests and the release of thousands of who they describe as "political prisoners" from Iraqi jails.

Many tribal leaders vowed back in June: "First we will fight Malaki, then we will fight IS."

Even though Malaki was forced to step down as prime minister in August, replaced by Haidar al-Abadi, tribes like Al-Bu Nimr – now the subject of a ferocious campaign of attacks by IS because of their willingness to fight back – are still in the minority.

"The important point about these mass killings is that they represent a distinct contrast with previous IS policy," Aymenn al-Tamimi, a Shillman-Ginsburg Fellow at the Middle East Forum, told Fairfax Media.

When IS took control of Fallujah, for example, it took steps to ensure local police and their families were not harmed when the city first fell out of government hands, he said.

"Instead emphasis was placed on the offer and process of repentance – that is, if you renounce fighting IS and lay aside your weapons, we will spare you, even if you have killed us in the past."

But it is not just Islamic State militants who are committing atrocities.

In a report released on Sunday, Human Rights Watch documented an attack on the Musab Bin Omair mosque on August 22 in which 34 people were killed by Iraqi pro-government militias and security forces.

According to accounts by five witnesses, including one survivor of the massacre in Diyala province armed men, some wearing civilian clothes and others in police uniforms, attacked the mosque, shooting to death 32 men, one woman, and one 17-year-old boy.

A survivor, who was inside the Sunni mosque, said he saw a man enter wearing the dark green T-shirt, pants and headband typically worn by Shiite militiamen affiliated with Asa'ib Ahl al-Haqq, a pro-government militia.

In response to a request for information about the attack, Interior Ministry Spokesman General Saad Ma'an Ibrahim said an "investigation commission" had been formed to examine the incident.

Human Rights Watch said the August 22 attack was consistent with a pattern of attacks including kidnappings and summary executions undertaken by Shia militias Asa'ib Ahl al-Haqq, the Badr Brigades, and Kita'ib Hezbollah in Baghdad, Diyala and Babel provinces.

"Iraq's international allies cannot allow the fight against the abusive extremists of ISIS to be carte blanche for the Iraqi government's allies to callously kill civilians who happen to be Sunnis," said Joe Stork, the deputy director of HRW's Middle East program.