Isis "may have committed all three of the most serious international crimes — namely war crimes, crimes against humanity and genocide," the United Nations human rights office said in a statement.
The agency published a horrifying report detailing killings, torture, rape, sexual slavery and the use of child soldiers by the extremists.
All of these crimes, it said, were violations of international human rights and humanitarian law, and some may amount to "crimes against humanity" and "war crimes".
The report, which is based on interviews with more than 100 witnesses and survivors of attacks in Iraq between June 2014 and February 2015, especially highlights brutal IS attacks on ethnic and religious groups, including Yazidis, Christians, Turkmen, Kurds and Shia.
Isis, which controls a swathe of territory in Iraq and neighbouring Syria, launched "a series of systematic and widespread attacks on the Yazidi minority's heartland in the northern Nineveh province last August.
According to the report, the attacks appeared intended "to destroy the Yazidi as a group," which "strongly suggests" Isis is guilty of "genocide" against the Yazidi.
Women as 'spoils of war'
In numerous Yazidi villages, men and boys over the age of 14 were rounded up and shot, while the women and girls were abducted as the "spoils of war".
The report, which was ordered by the UN Human Rights Council last September following a request from the Iraqi government, pointed out that some villages "were entirely emptied of their Yazidi population."
Many Yezidi women and girls were sold into sexual slavery or handed over to Isis members as "gifts", the report said, adding that witnesses had described hearing girls as young as six screaming for help as they were raped in a house
used by Isis fighters.
A pregnant 19-year-old had told the investigators she had been repeatedly raped by an Isis "doctor" over a period of two and a half months, and that he deliberately sat on her stomach, saying "this baby should die because it is an
infidel. I can make a Muslim baby."
Boys as young as eight were forced to convert to Islam and given religious and military training, including being forced to watch videos of beheadings, the report said.
Yazidis, whose ancient religion has elements of Christianity, Islam and Zoroastrianism, are considered to be devil worshippers by the Sunni Muslim militants.
Other religious and ethnic groups have also been targeted, according to the report, which points to the thousands of Christians who fled their homes last June after being ordered by Isis fighters to convert to Islam, pay a tax or
Also in June, Isis fighters attacked the Badoush prison, dividing the 3,000 inmates into groups, freeing the Sunnis and loading the remaining 600 mainly Shia inmates onto trucks, before driving them to a ravine and shooting them.
Some survivors told the investigators they had escaped being killed only because other bodies landed on top of them.
The jihadists have also ruthlessly targeted anyone perceived to be connected with the Iraqi government, the report said, pointing to the massacre last June of up to 1,700 cadets from the Speicher army base, after they reportedly surrendered.
A former police officer told the investigators Isis fighters had slashed the throats of his father, five-year-old son and five-month-old daughter after he showed his police ID card during a search.
Iraqi security forces and affiliated militia have also been accused of a range of serious crimes during their operations against Isis, the report said.
As the military campaign against the jihadists gained momentum last summer, militias seemed to "operate with total impunity, leaving a trail of death and destruction in their wake," it said.
The pro-government forces have carried out extra-judicial killings, torture, abductions and forcibly displaced large numbers of people, according to the report, which says they "may have committed war crimes."