Islamic State is holding an estimated 3,500 women and children as slaves in Iraq, the United Nations has said.
Those being held are primarily women and children from the Yazidi community, but a number are from other ethnic and religious minority groups, according to a report issued in Geneva.
The UN Assistance Mission for Iraq and the UN's human rights office have documented widespread abuses that "in some instances amount to war crimes, crimes against humanity, and possibly genocide".
They also said IS militants have imposed a harsh rule marked by gruesome public executions.
Methods of execution include shooting, beheading, bulldozing, burning people alive and throwing them off the top of buildings.
The UN said it had verified reports on child soldiers, saying that 800 to 900 youngsters had been abducted in the Iraqi city of Mosul for military and religious training.
Some were killed when they tried to escape fighting in western Anbar province.
Between the beginning of 2014 and the end of October 2015, it said that at least 18,802 civilians had been killed in Iraq, and another 36,245 wounded.
The UN's human rights chief, Zeid Ra'ad Al Hussein, said: "Even the obscene casualty figures fail to accurately reflect exactly how terribly civilians are suffering in Iraq.
"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 illustrated the "horror" that Iraqi refugees were attempting to escape from when they fled to Europe and other parts of the world.
IS seized large parts of Iraq in the summer of 2014, sweeping across northern and western areas.
However, in recent months Iraqi government forces have made advances against the group, taking back control of the city of Ramadi.