The Islamic State group executed 70 members of a Sunni tribe allied to the government in western Iraq earlier this week, a tribal leader and the United Nations said Wednesday.

The victims, members of the Albu Nimr tribe, were executed on Sunday in the Tharthar area north of Ramadi, the capital of the western Anbar province, tribal elder Naim Gaoud told AFP.

"These people who were executed were the fathers and brothers of members of the police, the army... and of tribal fighters who are battling Daesh," he said, using an Arabic acronym for IS.

"Daesh executed them by shooting," he said.

Iraqi security forces, backed by US-led coalition air strikes, launched a vast operation west of Ramadi Sunday to tighten the noose on IS, which captured the Anbar capital in May and controls most of the province.

Hatem al-Gaoud, another clan member reached by phone, said IS had trapped dozens of tribe members in the Khanzir area of Tharthar since the jihadist group launched its major offensive in Iraq last year.

"They gathered them outside Khanzir and shot them all in the head," he said.

"I don't know what IS did with the bodies, but it is likely they buried them in mass graves near the site of the execution," he said.

The UN Mission in Iraq's human rights office confirmed the mass execution.

"This is not the first attack on the Albu Nimr, since they have been actively opposed to ISIL (IS)," it said in an email to AFP.

Possibly as many as 300 of the tribe's members were killed around a year ago, when anti-IS forces were still holding out in some parts of Ramadi, which is the Albu Nimr's main hub.

The tribe was very active in the Awakening Councils, groups of Sunni tribal fighters the US military paid and armed a decade ago to fight against IS' previous incarnation in Iraq.

Local tribal fighters are seen by the United States as a key component of any successful effort to retake control of Anbar and other Sunni regions of Iraq IS took over virtually unopposed last year.

They no longer receive direct support from the US, whose assistance is channelled through the Shiite-dominated federal government.

IS has massacred hundreds of former Awakening fighters in an effort to intimidate Sunni residents from taking up arms against them.

Britain's Minister for the Middle East, Tobias Ellwood, said in a statement he was "deeply concerned and shocked" by reports of the Anbar executions.

"We have been clear, defeating ISIL will take time and patience but it is a fight we must win. They falsely use the name of Islam to commit atrocities and have once again slaughtered Sunnis, those they claim to be defending," he said.