Iraqi security forces and affiliated militias "may have committed some war crimes" while battling the insurgency, including killings, torture and abductions, the report said.
"Clearly international war crimes and crimes against humanity and possibly genocide appear to have been committed during this conflict. The genocide part relates particularly to the Yazidis," Hanny Megally, chief of the Asia, Pacific, Middle East and North Africa branch of the U.N. Human Rights Office, told a news briefing in Geneva.
"We are very keen to ensure that even as the conflict continues that evidence is preserved, protected and collected because that will be important for future accountability."
The U.N. investigators urged the Baghdad government to join the Hague-based ICC or pursue the crimes under domestic law.The U.N. Human Rights Council launched its inquiry in September after Islamic State, also known as ISIS, ISIL or IS, seized large swathes of northern and western Iraq.
There was a "manifest pattern of attacks" by Islamic State on Yazidis, viewed as "pagans" by IS, as well as Christians and other ethnic and religious minorities as its forces laid siege to towns and villages in Iraq, the report said.
"No community has been spared in Iraq from ISIL's violence….Essentially what we are seeing is the rich ethnic and religious diversity in Iraq that has been shattered completely," said Suki Nagra, chief U.N. investigator.
A "huge number of foreign fighters" were implicated in the atrocities, largely from neighbouring countries, but also a few Western states, she said, declining to be specific.
Unofficial estimates have put the number of Yazidis killed by IS militants in the hundreds. Nagra gave no figures on this, but said roughly 3,000 women, children and some men remained in IS custody. "This is an area that needs future investigation."
She added that mass graves were being uncovered in areas recently retaken by Iraqi government forces from Islamic State.
U.N. investigators also cited allegations that Islamic State had used chlorine gas, a prohibited chemical weapon, against Iraqi soldiers in the western province of Anbar in September.
Islamic State has treated captured women and children as "spoils of war", often subjecting them to rape or sexual slavery, the report added.
It said the insurgents' Islamic sharia courts in Mosul had meted out cruel punishments including stoning and amputation. "Thirteen teenage boys were sentenced to death for watching a football match," the report said.
_____________________________
By Stephanie Nebehay. Additional reporting by Isabel Coles in Iraq; Editing by Mark Heinrich. © Thomson Reuters 2015.http://www.reuters.com/article/2015/03/19/us-mideast-crisis-iraq-un-idUSKBN0MF0TX20150319