The United Nations human rights office said on Thursday that Islamic State fighters may have committed genocide against the minority Yezidi community in Iraq as well as crimes against humanity and war crimes against civilians including children.
In a report based on interviews with more than 100 alleged victims and witnesses, it urged the UN Security Council to refer the situation to the International Criminal Court (ICC) for prosecution of perpetrators.
The report also said Iraqi government forces and affiliated militias "may have committed some war crimes" while battling the insurgency.
The UN Human Rights Council launched its inquiry in September after the Islamist militant group Islamic State, also known as ISIS or ISIL, seized large swathes of northern Iraq.
The report said the Council had found "information that points to genocide, crimes against humanity and war crimes", and that the Security Council should "consider referring the situation in Iraq to the International Criminal Court".
There was a "manifest pattern of attacks" by Islamic State on Yezidis as well as Christians and other minorities as it laid siege to towns and villages in Iraq.
The U.N. investigators also cited allegations that ISIL had used chlorine gas, a prohibited chemical weapon, against Iraqi soldiers in the western province of Anbar in September.
Captured women and children were treated as "spoils of war", and often subjected to rape or sexual slavery, it said.
The report said that ISIL's Islamic sharia courts in Mosul had also meted out cruel punishments including stoning and amputation. "Thirteen teenage boys were sentenced to death for watching a football match," it said.
The U.N. investigators said it was "widely alleged" that Iraqi government forces had used barrel bombs, an indiscriminate weapon banned by international law, but this required further investigation.