Islamic State jihadists are committing such staggering atrocities in Iraq that they could amount to war crimes, the United Nations rights chief said Thursday.

"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).

Covering the period from July 6 to September 10, the 29-page study cited a litany of gross abuses and violations of international humanitarian law.

The jihadists were perpetrating their rights violations "with an apparent systematic and widespread character" -- the key criteria for abuses to amount to crimes against humanity, Zeid's spokesman Rupert Colville told AFP.

The jihadists have been committing "really every crime you can think of, and on a massive scale," he said, listing beheadings, abductions, torture and forcing women and young girls into sexual slavery as just a few examples.

IS has also recruited children into its ranks as combattants and routinely murders captured government soldiers and officials, the report showed, also slamming the desecration and destruction of religious and cultural sites.

On Wednesday, 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 of the country in early June.

Minorities have borne 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".

- 'Horrendous situation' -

"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.

Colville pointed out that while IS had halted its full-on attack on the Yazidi community, which forced tens of thousands to flee up Sinjar mountain in August, the jihadists were still singling out the minority, murdering men and forcing many women and girls into sexual slavery and forced marriages.

"It's so brazen. I mean ISIL are even giving some of these girls and women their mobile phones back so they can call their relatives so they can describe what's being done to them," he said.

The report also pointed to violations of the international laws of war and human rights by Iraqi security forces and affiliated militias -- including potentially indiscriminate and disproportionate air strikes and shelling.

Zeid called on Iraq to sign up to the International Criminal Court, thereby giving it jurisdiction to probe the "horrendous situation" in the country.

"This type of situation, where massive gross violations and abuses are taking place, including direct targeting of many thousands of civilians because of their religious or ethnic identity, is precisely why the International Criminal Court was created," he said.

Zeid, a Jordanian who took the helm of the UN's rights office last month, also pointed to a letter sent two weeks ago to the jihadists' leaders by scores of senior Muslim scholars from around the globe.

"It clearly states that in Islam it is forbidden to kill the innocent, or to kill emissaries, ambassadors and diplomats -- hence also journalists and aid workers.

"Torture and the re-introduction of slavery are also forbidden, as are forcible conversion, the denial of rights to women and a multitude of other acts," he said, accusing the group of committing such crimes on a "daily basis".