It said ISIS fighters had committed gross human rights violations of an 'increasing sectarian nature' against groups including Christians, Yazidis and Shi'ite Muslims in a widening conflict that has forced 1.8 million Iraqis to flee their homes.
UN Investigators believe as many as 2,500 women and children have been captured, subjected to sex attacks and then sold for around $10 a head by extremist militants in Iraq.
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Barbaric: A UN report has confirmed a series of atrocities committed by the Islamic State including this mass execution of around 1,500 Iraqi soldiers and security officers in the Salahuddin province of Iraq in June
However, the report also said Iraqi government air strikes on the Muslim militants had caused 'significant civilian deaths' by hitting villages, a school and hospitals in violation of international law.
UN High Commissioner for Human Rights Zeid Ra'ad al Hussein said: '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.'
In a statement, he called again for the Baghdad government to join the International Criminal Court, saying the Hague court was set up to prosecute such massive abuses and direct targeting of civilians on the basis of their religious or ethnic group.
There have been reports of hundreds of Yazidi women and children being sold at jihadi slave markets. In this picture a displaced Yazidi girl takes shelter at a school in Dahuk
According to investigators, slave markets have been set up in Raqqa, Syria and the al-Quds area of Maturat in Iraq partly to attract new Islamic State fighters. This Yazidi woman, holding a child, is shown taking shelter in a school after being displaced from her home
The report said the ISIS atrocities 'include attacks directly targeting civilians and civilian infrastructure, executions and other targeted killings of civilians, abductions, rape and other forms of sexual and physical violence perpetrated against women and children, forced recruitment of children, destruction or desecration of places of religious or cultural significance, wanton destruction and looting of property, and denial of fundamental freedoms.'
In a single massacre on June 12, about 1,500 Iraqi soldiers and security officers from the former U.S. Camp Speicher military base in Salahuddin province were captured and killed by Islamic State fighters, according to the 29-page report by the UN Human Rights Office and the U.N. Assistance Mission for Iraq (UNAMI).
However, the bodies have not been exhumed and the precise toll is not known.
Persecuted: The report also said ISIS snatched hundreds of women and girls, predominantly from the Yazidi (above) and Christian communities, and took them to Syria as a reward or to be sold as sex slaves
No one disputes that Iraqi military recruits were led off the base near Tikrit unarmed and then machine gunned in their hundreds into mass graves by Islamic State, whose fighters boasted of the killings on the Internet.
Women have been treated particularly harshly, the report said: 'ISIL (has) attacked and killed female doctors, lawyers, among other professionals.'
In August, it said, ISIS took 450-500 women and girls to the Tal Afar citadel in Iraq's Nineveh region where '150 unmarried girls and women, predominantly from the Yazidi and Christian communities, were reportedly transported to Syria, either to be given to ISIS fighters as a reward or to be sold as sex slaves'.
According to investigators, slave markets have been set up in Raqqa, Syria and the al-Quds area of Maturat in Iraq partly to attract new Islamic State fighters.
The Times reports that the UN has been contacted by captured women who had managed to keep their mobile phones and reported being sexually abused.
The newspaper says that 500 women were reported as having been taken when a Yazidi village in northwest Iraq was overrun by militants.
It quotes a Euronews interview with a 19-year-old mother whose husband was shot by Isis fighters as they swept through the Sinjar area in northern Iraq before she was sold in Mosul.
The woman, named as Amsha, is reported as saying: One Yazidi woman was given to 10 Muslim ISIL men. We were sold for $10 or $12. Who could accept that behaviour? Can God accept that?'
But the UN report also voiced deep concern at violations committed by the Baghdad government and allied fighters, including air strikes and shelling that may not have distinguished between military targets and civilian areas.
At least 9,347 civilians had been killed and 17,386 wounded so far through September, well over half of them since the Islamist insurgents began seizing large parts of northern Iraq in early June, the UN said.
Turkish army tanks are transported close to the Syrian border as its parliament prepare to vote on whether to vote to allow its military to enter Iraq and Syria as well as letting foreign troops to use its territory against ISIS
A Turkish soldier holds a lost baby as he looks for its mother as thousands of new Syrian refugees arrive in Turkey from the town of Kobani which is under siege from ISIS militants
Islamic State and allied groups have attacked and destroyed places of religious and cultural significance in Iraq that do not conform to its 'takfiri' doctrine, the U.N. report said, referring to the beliefs of Sunni militants who justify their violence by branding others as apostates.
Islamic State pushed on with its assault on a Syrian border town today despite coalition air strikes meant to weaken them, sending thousands more Kurdish refugees into Turkey and dragging Ankara deeper into the conflict.
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