(CNN) Nearly 19,000 civilians were killed in Iraq between January 2014 and October 2015 -- a toll the United Nations calls "staggering" in a new report.

Much of the suffering was attributed to ISIS, the brutal Islamist terror group which has declared an Islamic caliphate across the vast stretches of territory it holds in Iraq and neighboring Syria.

The report also estimated that ISIS holds about 3,500 slaves and said it continues to subject women and children to sexual violence , particularly in the form of sexual slavery.

Other numbers in the report are mind-boggling. In the 21-month period in Iraq:

• At least 18,802 civilians were killed, about half of them in Baghdad.

• Another 36,245 were injured.

• About 3.2 million people were internally displaced, including a million school-aged children.

The actual figures could be much higher, the report said.

ISIS crimes: 'Possibly genocide'

The report also documented alleged abuses by Iraqi security forces and allied groups fighting ISIS, but attributed much of the suffering to ISIS itself.

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"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 report said, using another name for ISIS

The group, which has controlled Iraq's second largest city, Mosul, since June 2014, employed horrifying methods of killing, the report said, including beheading, bulldozing, burning alive and throwing people off the tops of buildings.

However, improvised explosive devices -- including explosives worn by suicide bombers and those carried in vehicles -- were the deadliest tactic used against civilians, it said.

The report was prepared by the U.N. Assistance Mission for Iraq and the Office of the High Commissioner for Human Rights and was based largely on testimony obtained directly from survivors or witnesses of rights violations, including interviews with internally displaced people.

3,500 slaves held by ISIS, report says

The report noted that ISIS continued to target ethnic and religious minorities, such as the Yazidis , systematically persecuting them with the apparent goal of suppressing, permanently expelling or destroying the groups.

mainly women and children, The 3,500 slaves that ISIS is estimated to have,mainly women and children, came primarily from the persecuted Yazidi community , but a number hailed from other ethnic and religious minorities, it said.

Report: Women killed for refusing sex

The report detailed a number of specific incidents regarding the enslavement of women in Iraq.

In May, it said, a 24-year-old Yazidi woman was killed while trying to escape from ISIS captivity in Nineveh province, her body left on the side of the road.

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In June, 42 Yazidi women were abducted from the same region and taken to ISIS-held Deir Ezzor in Syria, it said. Some of the women were reportedly sold to ISIS fighters there for $500 to $2,000, it said.

And in August, ISIS reportedly killed 19 women in Mosul for refusing to have sex with its fighters.

The report also documented an ISIS-run Quran recitation contest in the city, which advertised sex slaves as prizes for the winners.

The report's authors also referred to verified reports that 800 to 900 children in Mosul had been abducted by the terror group for religious education and military training.

There were also reports that child soldiers who had fled fighting on the front lines in Anbar had been killed, it said.

Brutal Sharia punishments

The report said that ISIS targeted those perceived to be opposed to its ideology and rule; people affiliated with the government, including police officers and public officials; doctors; lawyers; journalists; and tribal and religious leaders.

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Many were then sentenced in ISIS-run courts, where they faced punishments including death, stoning or amputation.

A number of mass graves had been uncovered in Iraq, the report said.

World's support needed

U.N. High Commissioner for Human Rights Zeid Ra'ad Al Hussein warned that the civilian death toll may be much higher and said action is urgently needed to deal with a situation where most perpetrators of violence face no consequences.

"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 said.

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"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."

The report's authors also noted "concerning" reports of unlawful killings and abductions by groups associated with Iraq's government, which may have been reprisals against people perceived to be supporting the terror group, it said.

"Despite their steady losses to pro government forces, the scourge of ISIL continues to kill, maim and displace Iraqi civilians in the thousands and to cause untold suffering," said Jan Kubis, the U.N. secretary-general's special representative for Iraq.

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He called on the international community to enhance its support to the Iraqi government's stabilization and reconstruction efforts in areas liberated from ISIS, "so that all Iraqis displaced by violence can return to their homes in safety and in dignity and that affected communities can be reestablished in their places of origin."