Islamic State jihadists may have committed genocide against the Yazidi minority in Iraq as well as crimes against humanity and war crimes on other civilians in territory the group controls, the UN said in a report released Thursday.

The report by the United Nations High Commissioner for Human Rights (UNHCR) urged the Iraqi government to join the International Criminal Court and to accept the Hague-based court's jurisdiction over the crimes cited in Thursday's release.

The Islamic State group, which controls a stretch of territory in Iraq and neighbouring Syria, "may have committed all three of the most serious international crimes — namely war crimes, crimes against humanity and genocide", the UNHCR said in a statement.

The report detailed killings, torture, rape, the sexual enslavement of women and children, and the use of child soldiers by the extremist group, which is also known as ISIS or ISIL.

Based on interviews with more than 100 witnesses and survivors of attacks in Iraq between June 2014 and February 2015, the report highlights the Islamic State group’s brutal attacks on ethnic and religious groups including the Yazidis, Christians, Turkmen, Kurds and Shiites.

"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," chief UN investigator Suki Nagra told a news briefing in Geneva.

The Islamist group launched "a series of systematic and widespread attacks” on the Yazidi minority's heartland in northern Nineveh province last August. The pattern of its attacks against the Yazidi illustrated “the intent of ISIL to destroy the Yazidi as a group”, the report says, adding that this “strongly suggests” that the group committed genocide.

Genocide is defined under international law as “acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group”.

The Vatican's envoy to the UN in Geneva warned in March that Islamic State fighters were committing genocide and that the world needed to intervene.

"We have to stop this kind of genocide," Archbishop Silvano Tomasi told the Catholic website Crux. "Otherwise we'll be crying out in the future about why we didn't do something, why we allowed such a terrible tragedy to happen."

The move was unusual because the Vatican has traditionally opposed the use of force.

'This baby should die'

Tens of thousands of Yazidis fled into the mountains as the Islamists advanced on the town of Sinjar last August.

Captured women and children were treated as “spoils of war” and often subjected to rape or sexual slavery, the report said.

Many Yazidi women and girls were sold or handed over to Islamist fighters as "gifts", the report said. Witnesses described hearing girls as young as 6 years old screaming for help as they were raped in a house by Islamic State fighters.

A pregnant 19-year-old told investigators that she had been repeatedly raped by an Islamic State "doctor" over a period of two and a half months. He deliberately sat on her stomach, saying, "This baby should die because it is an infidel. I can make a Muslim baby."

The UN's Nagra said on Thursday that roughly 3,000 women, children and some men are still being held by the militants.

The Islamic State group’s sharia law courts in the Iraqi city of Mosul have also meted out cruel punishments, including stonings and amputations.

“Thirteen teenage boys were sentenced to death for watching a football match,” the UN said.

Tens of thousands of Christians fled their homes in June and July after being ordered by Islamic State fighters to convert to Islam, pay a tax or leave.

Islamic State militants attacked the Badoush prison in June, dividing the 3,000 inmates into groups. They then freed the Sunnis and loaded the remaining 600 mainly Shiite inmates onto trucks before driving them to a ravine and shooting them.

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

The jihadists have ruthlessly targeted anyone perceived to be connected with the Iraqi government, the report said, citing the massacre last June of up to 1,700 cadets from the Speicher army base after they reportedly surrendered.

A former police officer told investigators that jihadist fighters had slashed the throats of his father, his 5-year-old son and his 5-month-old daughter after he showed his police ID card during a search.

But the report also said that Iraqi government forces and its affiliated militias “may have committed some war crimes” while battling the Islamic State group. As the military campaign against the jihadists gained momentum last summer, militias seemed to "operate with total impunity, leaving a trail of death and destruction in their wake", it said.

The UN investigators said it was “widely alleged” that Iraqi government forces had used barrel bombs, a weapon banned by international law, but that the allegations required further investigation.

Pro-government forces have also carried out extra-judicial killings, torture, abductions and forcibly displaced large numbers of people, according to the report.

(FRANCE 24 with REUTERS, AFP)