More than 18,000 civilians were killed in Iraq between January 1, 2014, and October 31, 2015, according to the United Nations, which says violence there remains "staggering."
An estimated 3,500 people, mainly women and children, are believed to be held as slaves and 3.2 million people have become internally displaced, it said.
A UN report released on Tuesday said the Islamic State (IS or ISIL) continued to perpetrate systemic and widespread atrocities that could amount to war crimes and possibly genocide.
IS was killing and abducting people opposed to their ideology, people affiliated with the government, and professionals such as doctors, lawyers and journalists, as well as tribal and religious leaders. About half the deaths took place in Baghdad.
The report, which is based largely on the testimony of victims, survivors, and witnesses, describes how IS made "gruesome public spectacles" out of its killings in the form of "shooting, beheading, burning alive, bulldozing, and throwing off buildings."
Women and children were subjected to sexual violence and sexual slavery, it added.
Those slaves "come primarily from the Yazidi community, but a number are also from other ethnic and religious minority communities," said the report compiled by the UN Assistance Mission for Iraq and the Office of the High Commissioner for Human Rights.
The report also documented alleged abuses by Iraqi government security forces and associated forces, including unlawful killings and abductions.
"Some of these incidents may have been reprisals against persons perceived to support or be associated with ISIL," the document states. "Moreover, as civilians move around the country, fleeing violence, they have continued to face government restrictions on their ability to access safe areas."
'This report starkly illustrates what Iraqi refugees are attempting to escape when they flee to Europe'
Once civilians reached safe areas, some were arbitrarily arrested and others were forcibly expelled, it said, raising concerns the government was not taking all feasible precautions to protect the civilian population.
The report also documents the discovery of various mass graves, in areas regained by the government from IS, as well as graves from Saddam Hussein's era.
The Special Representative of the UN Secretary-General for Iraq, Jan Kubis, called on all parties to the conflict to protect civilians, and appealed to the international community to further its support for humanitarian and reconstruction efforts in areas liberated from IS.
Despite the jihadi group's steady losses, IS "continues to kill, maim and displace Iraqi civilians in the thousands and to cause untold suffering," he said.
The report did not even tell the full horrifying story, said UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein.
"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," Hussein said.
"This report lays bare the enduring suffering of civilians in Iraq and starkly illustrates what Iraqi refugees are attempting to escape when they flee to Europe and other regions. This is the horror they face in their homelands."