Nearly 19,000 Iraqi civilians have been killed and 36,000 wounded since 2014 during the ongoing conflict in the country, according to a new UN report.
Another 3.2 million people have been internally displaced, including more than a million children of school age.
The joint report was compiled by the UN Assistance Mission for Iraq (UNAMI) and the Office of the High Commissioner for Human Rights (OHCHR) and covers the period from January 2014 until the end of October 2015.
"The violence suffered by civilians in Iraq remains staggering," said the report, issued in Geneva.
"The so-called ‘Islamic State of Iraq and the Levant’ (ISIL) continues to commit systematic and widespread violence and abuses of international human rights law and humanitarian law.
"These acts may, in some instances, amount to war crimes, crimes against humanity, and possibly genocide."
The UN said an estimated 3,500 people, mainly women and children, are believed to be held as slaves in Iraq by so-called Islamic State militants who impose a harsh rule marked by gruesome public executions.
"Those being held are predominantly women and children and come primarily from the Yazidi community, but a number are also from other ethnic and religious minority communities," the report said.
It detailed executions by shooting, beheading, bulldozing, burning alive and throwing people off the top of buildings.
The report 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.