An estimated 3,500 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 said on Tuesday.The militant group, which also controls large parts of neighboring 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 3,500 people were "currently being held in slavery by ISIS.""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.A pamphlet titled "Questions and Answers regarding slave-holding," distributed to ISIS fighters in October 2014, defines slave women as part of the loot the organization's fighters are allowed to take following their victories over "the enemies of Islam." According to the pamphlet, the religion authorizes the rape of infidel slave women, but the issue of raping Muslim apostates is considered controversial. Since the slave women are regarded as the private property of the fighters who captured them, ISIS fighters are also allowed to buy and sell them. If an ISIS fighter that holds slaves dies, his family inherits them. The report detailed executions by shooting, beheading, bulldozing, burning alive and throwing people off the top of buildings.It said the United Nations 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."