Amnesty International has called for an investigation into possible war crimes in the battle against Islamic State (IS) militants in northern Iraq.
Both the Kurdish and Islamic fighters are suspected of war crimes.
The UN said violence suffered by civilians in Iraq "remains staggering", with at least 18,800 killed between 1 January 2014 and 31 October 2015.
Some 3.2 million people have also been displaced internally over the same period, according to a new report.
The UN accused so-called IS of systematic and widespread violence, including holding some 3500 people, mainly women and children, as slaves.
Alleged abuses by troops, militiamen and Kurdish forces were also recorded.
UN High Commissioner for Human Rights Zeid Raad Al Hussein said the report "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, by the United Nations Assistance Mission for Iraq and the Office of the High Commissioner for Human Rights, was based on interviews with the displaced and direct testimony from victims, survivors or witnesses of rights abuses.
It said the figures of 18,802 civilians killed and 36,245 wounded between 1 January 2014 and 31 October 2015 could be much higher.
In particular, the figures from the conflict-torn western province of Anbar, much of which is held by IS, might not fully reflect the situation.
About half of the deaths were recorded in Baghdad province and improvised explosive devices (IEDs) were the deadliest tactic used against civilians.
Nevertheless, the figures for civilian deaths are still significantly below those recorded at the height of the sectarian insurgency in Iraq in 2006 and 2007.
Documented cases of IS abuse in 2015
- 10 July: IS militants force victims to lie down in central Mosul in front of large crowd. A bulldozer is driven over them
- 4 June: Two young males and a 60-year-old man thrown from a building in Ninewa for alleged homosexual acts
- 1-2 August: IS militants allegedly kill 19 women in Mosul for refusing to have sex with fighters
- 21 June: Women offered as sex slaves to the top three winners of a Koran memorisation competition in Mosul
- 23 June: IS group video shows men placed in a car and hit by rocket-propelled grenade; men drowned in a cage; men decapitated with explosives
- 12-15 July: Four imams in Mosul accused by self-appointed IS court of conducting forbidden taraweeh Ramadan prayers. Shot in the head