An estimated 3500 people, mainly women and children, are believed to be held as slaves in Iraq by Islamic State militants who impose a harsh rule marked by gruesome public executions, the United Nations has claimed.

The militant group, which also controls large parts of neighbouring Syria, has committed widespread abuses that may “in some instances, amount to war crimes, crimes against humanity, and possibly genocide”, the report said.

The UN Assistance Mission for Iraq and the UN human rights office estimated that 3500 people were “currently being held in slavery by ISIL”.

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“Those being held are predominantly women and children and come primarily from the Yezidi community, but a number are also from other ethnic and religious minority communities,” said the joint report issued in Geneva on Tuesday.

Jinan, 18, is one of the lucky Yazidi women to survive IS capture. Photo: Getty

The report detailed executions by shooting, beheading, bulldozing, burning alive and throwing people off the top of buildings.

It said the UN had information about the murder of child soldiers and had verified reports suggesting between 800 and 900 children in Mosul had been abducted for military and religious training.

“Even the obscene casualty figures fail to accurately reflect exactly how terribly civilians are suffering in Iraq,” UN human rights chief Zeid Ra’ad Al Hussein said in a statement.

“The figures capture those who were killed or maimed by overt violence, but countless others have died from the lack of access to basic food, water or medical care.”

He added that the report laid bare the “horror” that Iraqi refugees were attempting to escape when they fled to Europe and other regions.