"The array of violations and abuses perpetrated by ISIL and associated armed groups is staggering, and many of their acts may amount to war crimes or crimes against humanity," said UN human rights chief Zeid Ra'ad Al Hussein.
Zeid's comments came as his office and the UN's monitoring mission in Iraq released a joint report on the offensive carried out by the Islamic State (IS) group, also know as the Islamic State of Iraq and the Levant (ISIL).
It said they were being perpetrated by IS and associated armed groups "with an apparent systematic and widespread character."
Among them were attacks on civilians, including executions and looting, as well as the murder of captured government soldiers and officials.
More From This Section
Yesterday, the UN Assistance Mission in Iraq announced that at least 9,347 civilians have been killed so far in 2014, and 17,386 wounded, well over half of them since the jihadists began overrunning swathes of the north in early June.
It also slammed the desecration and destruction of religious and cultural sites.
Minorities have born the brunt of the Sunni jihadists' attacks, with Turkmen, Shabak, Christians, Yazidi, Kurds and Shiite Muslims targeted for attacks "aimed at destroying, suppressing or cleansing them from areas under their control".
"This report is terrifying," said Nickolay Mladenov, UN special envoy for Iraq.
He noted that hundreds of alleged killings of civilians were not included because they had not yet been adequately verified.
Zeid called on Iraq to sign up to the International Criminal Court, thereby giving it jurisdiction to probe the "horrendous situation" in the country.