The U.N. estimates that the group holds about 3,500 slaves. The slaves are mainly women and children of the Yazidi religion. Some of the women are killed for trying to escape or for refusing sexual relations with Islamic State fighters.
The report said that 800 to 900 children in Mosul have been abducted and put through Islamic State religious and military training. There have been accounts of child soldiers who were killed for fleeing fighting on the front lines of Iraq's Anbar province.
Islamic State courts have sentenced their opponents to punishments including death, stoning, or amputation. Those targeted included people affiliated with the government, doctors, lawyers, journalists, and tribal and religious leaders.
The report said actual casualties could be "much higher than reported" due to difficulties in verifying incidents. This is true particularly of Anbar province, in Iraq's west, much of which is controlled by the Islamic State.
The Islamic State group has targeted ancient sites, churches, mosques, shrines, and tombs it considers to be un-Islamic.
U.N. agencies have also received reports of human rights violations and abuses by pro-government forces. These include unlawful killings, abductions, movement restrictions, and forced evictions. Military airstrikes, shelling and other operations have killed civilians and damaged their property.
At least 2,365 civilians were killed by unknown perpetrators in 2015 from May 1 to October 31.
The U.N. report noted new discoveries of mass graves. Many of the mass graves are recent, while some date to the time of Saddam Hussein.
Jan Kubis, the Special Representative of the U.N. Secretary-General for Iraq, especially denounced the Islamic State group's treatment of civilians.
"I strongly reiterate my call to all parties to the conflict to ensure the protection of civilians from the effects of violence," he said.