In a new report, the UN also estimates 3,500 people, mainly women and children, are being held as slaves by IS militants in Iraq.
The United Nations describes the levels of violence against civilians in Iraq as staggering.
A UN report documents a wide range of human rights abuses by the self-proclaimed Islamic State militants, including executions by shooting, public beheading, bulldozing and burning alive.
It reveals around 3,500 people, mostly women and children, are being held as slaves by IS in Iraq.
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They're believed to be mainly from the Yazidi religious minority captured in 2014 and forced into sexual slavery.
UN spokeswoman Ravina Shamdasani says the horrors that the people of Iraq are facing are tremendous.
"You got people being bulldozed, you got people being decapitated and hanged in public to set an example for the others, that this is what will happen to you if you do not obey. The kinds of violations that we have documented committed by ISIL could amount to war crimes, crimes against humanity and possibly genocide."
The report also shows more than 18,000 civilians have been killed in Iraq since January 2014 as Iraqi forces battled IS.
It reveals 800 to 900 children were abducted from Iraq's second largest city, Mosul, for religious and military training.
Ravina Shamdasani says a number of IS child soldiers were killed by the extremists when they tried to flee.
"ISIL is abducting young children and recruiting them, putting them in the frontlines of war, and in one case that we have documented, these children have been fled the frontlines of war because they were scared. When they got back, they were executed by ISIL for desertion. These are the kinds of horrors that people are facing, women have been subject to sexual slavery, severe restrictions are being placed on their movement and on the movement of men."
Francesco Motta is the Director of the UN Assistance Mission for Iraq, UNAMI.
He says it's difficult to accurately reflect the extreme suffering faced by civilians in Iraq.
"We have received reports of disappearances, harassment, threats and intimidation of the local civilian population but also of individuals who have been displaced, sometimes entailing forced relocations from places where they have sought refuge to other locations but also at times preventing civilians from returning voluntarily to their homes."
Mohammad Ali Al-Nsour is the UN Human Rights's representative in the Middle East and North Africa Region.
He says there are steps the Iraqi government can take immediately.
"Basic recommendations for the Iraqi government (are) to accept the jurisdiction of the international court and to apply the Rome statute (of the International Criminal Court) and the domestic and national courts. And to start, immediately, the accountability for the perpetrators of these violations."
The UN's Ravina Shamdasani says the report lays bare the horrors that cause Iraqis to flee to Europe and other regions.
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