Intentional war crimes and "possibly genocide" may have been committed against Yazidis in Iraq by Islamic State militants, the UN said today.

A new report based on interviews with more than 100 survivors of attacks between June 2014 and February 2015 urges the UN Security Council to refer the issue to the International Criminal Court.

One witness told investigators that ISIS militants laughed as two teenage girls were raped in the next room. A pregnant woman said an ISIS 'doctor' repeatedly raped her, and sat on her stomach telling her: "this baby should die because it is an infidel; I can make a Muslim baby."

"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 reporters.

"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."

According to the report, ISIS militants had "the intent...to destroy the Yazidi as a group" and undertook a "manifest pattern of attacks". This "strongly suggests" that militants may have perpetrated genocide.

A minority ethno-religious group in Iraq, Yazidis have been systematically persecuted by Islamic State during its attempts to create a caliphate. Their religion is an offshoot from Zoroastrianism, which blends ancient religious traditions with both Christianity and Islam. IS insurgents believe them to be "devil-worshippers".

Hundreds of Yazidis were killed and thousands more captured, made slaves and raped last year. Christians and other ethnic and religious minority groups have also experienced considerable persecution under ISIS.

"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 chief UN investigator Suki Nagra.

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.

ISIS has treated captured women and children as "spoils of war", often subjecting them to rape or sexual slavery, the report added. Last year it was revealed that hundreds of Yazidi men, women and children were being held in "makeshift detention facilities" in Iraq and Syria. Many had been forced to convert to Islam, while others had been sexually abused, raped, and girls in their teens taken as wives for jihadists. Girls as young as one were being sold for the equivalent of £100.

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 ISIS 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.

The report also said that Iraqi security forces and affiliated militias "may have committed some war crimes" while battling the insurgency.

Militias seemed to "operate with total impunity, leaving a trail of death and destruction in their wake" as they mounted a campaign against ISIS, it claimed.

Additional reporting by Reuters