IS "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".
IS, 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.
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According to the report, the attacks appeared intended "to destroy the Yazidi as a group," which "strongly suggests" IS is guilty of "genocide" against the Yazidi.
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".
Many Yezidi women and girls were sold into sexual slavery or handed over to IS 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 IS fighters.
A pregnant 19-year-old had told the investigators she had been repeatedly raped by an IS "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.