Geneva: United Nations human rights chief Navi Pillay on Monday condemned "appalling, widespread" crimes being committed by Islamic State forces in Iraq, including mass executions of prisoners and "ethnic and religious cleansing".
The persecution of entire communities and systematic violations by the Al Qaeda offshoot, documented by UN human rights investigators, would amount to crimes against humanity and war crimes under international law, she said in a statement.
"Grave, horrific human rights violations are being committed daily by IS and associated armed groups," Pillay said, citing targeted killings, forced conversions, abductions, slavery, sex crimes, forced recruitment and destruction of places of worship.
Christians, Yazidis and Turkmen were among the minorities targeted by the minority sect militant group, which has forced people to convert to their strict form of Sharia law, she said.
Islamic State insurgents have captured a third of Iraq with little resistance and declared a caliphate in the areas of Iraq and Syria it controls. It has drawn the first American air strikes in Iraq since the end of the occupation in 2011.
Some 1.2 million people have fled fighting and IS's advance in Iraq this year, the U.N. refugee agency says.
The Al Qaeda splinter group seized control of the city of Mosul on June 10, in a spectacular show of strength against the Baghdad government.
IS loaded 1,000 to 1,500 prisoners from Badush prison in Mosul onto trucks and took them to a vacant area for screening, Pillay said.
Minority sect inmates were taken away again on the trucks.
"IS gunmen then yelled insults at the remaining prisoners, lined them up in four rows, ordered them to kneel and opened fire," she said.
Up to 670 prisoners from Badush prison were killed by Islamic State on June 10, she said, quoting dozens of survivors and witnesses, some of whom survived by pretending to be dead.
"Such cold-blooded, systematic and intentional killings of civilians, after singling them out for their religious affiliation, may amount to war crimes and crimes against humanity," said Pillay, a former UN war crimes judge who steps down on August 31 after serving six years as UN rights boss.
In northern Nineveh province, hundreds of Yazidis were killed and up to 2,500 kidnapped in early August, Pillay said, citing testimony from victims and witnesses.
Yazidis fled their ancient homeland of Sinjar and other villages to escape the militants.
Those who agreed to convert are being held by IS, but witnesses report that among those who refused, "men were executed while the women and their children were taken as slaves and either handed over to IS fighters as slaves or threatened with being sold", the UN statement said.
IS also killed and abducted hundreds of Yazidis in Cotcho village in southern Sinjar on August 15, Pillay said, citing witness testimony including "harrowing phone calls".
UN human rights investigators have received increasing reports of civilians being targeted for killing, she said, citing incidents of dozens being killed in Basra and Diyala.