GENEVA - The United Nations said on Thursday Islamic State forces may have committed genocide in trying to wipe out the Yazidi minority in Iraq as well as war crimes against civilians including children.

In a report based on interviews with more than 100 alleged victims and witnesses, the UN Human Rights Office urged the UN Security Council to refer the issue to the International Criminal Court (ICC) to prosecute perpetrators, including foreign members of the ultra-radical insurgent group.

Iraqi security forces and affiliated militias “may have committed some war crimes” while battling the insurgency, including killings, torture and abductions, the report said.

“Clearly international war crimes and crimes against humanity and possibly genocide appear to have been committed during this conflict. The genocide part relates particularly to the Yazidis,” Hanny Megally, chief of the Asia, Pacific, Middle East and North Africa branch of the UN Human Rights Office, told a news briefing in Geneva.

“We are very keen to ensure that even as the conflict continues that evidence is preserved, protected and collected because that will be important for future accountability.” The UN investigators urged the Baghdad government to join the Hague-based ICC or pursue the crimes under domestic law.

The UN Human Rights Council launched its inquiry in September after Islamic State, also known as ISIS, ISIL or IS, seized large swathes of northern and western Iraq.

There was a “manifest pattern of attacks” by Islamic State on Yazidis, viewed as “pagans” by IS, as well as Christians and other ethnic and religious minorities as its forces laid siege to towns and villages in Iraq, the report said.

“No community has been spared in Iraq from ISIL’s violence....Essentially what we are seeing is the rich ethnic and religious diversity in Iraq that has been shattered completely,” said Suki Nagra, chief UN investigator. A “huge number of foreign fighters” were implicated in the atrocities, largely from neighbouring countries, but also a few Western states, she said, declining to be specific.

Unofficial estimates have put the number of Yazidis killed by IS militants in the hundreds. Nagra gave no figures on this, but said roughly 3,000 women, children and some men remained in IS custody. “This is an area that needs future investigation.”

She added that mass graves were being uncovered in areas recently retaken by Iraqi government forces from Islamic State.

UN investigators also cited allegations that Islamic State had used chlorine gas, a prohibited chemical weapon, against Iraqi soldiers in the western province of Anbar in September.

Islamic State has treated captured women and children as “spoils of war”, often subjecting them to rape or sexual slavery, the report added.

It said the insurgents’ Islamic sharia courts in Mosul had meted out cruel punishments including stoning and amputation. “Thirteen teenage boys were sentenced to death for watching a football match,” the report said.

Meanwhile, Iraq has not requested air support from the US-led coalition for its campaign to retake Tikrit from Islamic State insurgents, a senior military official in the coalition said on Thursday, as the assault on the city remained on pause for nearly a week.

Some Iraqi officials this week said more air strikes are needed to dislodge the militants, who are holed up in a vast complex of palaces built when Saddam Hussein was in power and have turned the city into a labyrinth of homemade bombs and booby-traps.

The coalition has been conspicuously absent from the campaign - the largest to be undertaken by Iraqi forces and Iranian-backed militia groups since Islamic State overran a third of the country last summer.

“We have not been asked by the Iraqi government to conduct air strikes in Tikrit,” said the senior military official in the coalition, declining to speculate why. “We don’t conduct any strikes without the request and agreement of the government of Iraq or the Kurdistan Regional Government”.

More than 20,000 troops and militiamen are taking part in the offensive, which began more than two weeks ago, supported by a relatively small contingent of fighters from Tikrit and the surrounding area.

Having made steady progress towards Tikrit, retaking the surrounding towns before entering the city itself last week, the offensive has slowed and there have been no major advances since Friday.

Iraqi officials say there is no doubt they will prevail in Tikrit, but they have paused to await reinforcements, limit casualties in their ranks, and give remaining civilians one last chance to leave.

The senior military official in the coalition said it was “absolutely normal” that Iraqi forces should stop to regroup before a final assualt on the militants, now cornered in an area bounded by the river Tigris to the east.

Asked about the coalition’s view on the involvement of militias in Tikrit and elsewhere, the senior official said it was up to Baghdad which forces took part.

“I think any force that is here and the government of Iraq uses in the fight against IS (Islamic State) is a good thing,” the official said. “It’s been quite effective.”