There was "strong evidence" fighters from Islamic State and linked groups had carried out targeted killings, forced conversions, abductions, sexual abuse and torture, UN Deputy High Commissioner for Human Rights Flavia Pansieri told the session.
Iraqi police have executed detainees while Iraqi soldiers have shelled towns and carried out air strikes killing and injuring many civilians, Ms Pansieri said, as she opened the meeting of the UN Human Rights Council in Geneva.
She added: "Systematic and intentional attacks on civilians may constitute war crimes and crimes against humanity. Individuals, including commanders, are responsible for these acts."
Ms Pansieri voiced deep concern at persecution of Christians, Yazidis, Shia, Turkmen and other ethnic groups by Islamic State forces that have swept through western and northern Iraq since June, driving 1.2 million Iraqis from their homes.
She said: "The reports we have received reveal acts of inhumanity on an unimaginable scale. I am particularly concerned about the persecution that is taking place."
The one-day session was called by Iraq with the support of allies including the US. The 47-member state forum is expected to agree to Baghdad's request to send a team of UN experts to investigate crimes committed by Islamic State and others in the conflict.
At least 1,420 people were killed in Iraq last month, new figures published by the UN showed yesterday, as sectarian violence raged in the centre and north of the country.
Another 1,370 Iraqis were wounded and 600,000 people forced to flee as Islamic State militants, who have grabbed large areas of territory since June, pushed into land controlled by Kurdish troops and targeted religious minorities.
The UN said the casualty figures could be far higher.