The UN says it has received reports from Iraq that “reveal acts of inhumanity on an unimaginable scale”.
Deputy Human Rights Commissioner Flavia Pansieri said Islamic State (IS) was believed to have committed systematic and intentional attacks on civilians.
They include targeted killings, forced conversions, slavery, sexual abuse, and the besieging of entire communities.
The UN Human Rights Council has agreed to send an emergency mission to investigate the IS crimes.
The unrest in Iraq has escalated dramatically in recent months as Islamic State, formerly known as Isis, and allied Sunni rebels have taken control of large parts of northern and western Iraq.
Thousands of people have been killed, the majority of them civilians, and more than a million others have been forced to flee their homes.
On Monday, the UN Human Rights Council adopted a resolution that will allow a team to investigate whether war crimes and crimes against humanity are being committed in Iraq.
“We are facing a terrorist monster,” Iraqi Human Rights Minister Mohammed Shia al-Sudani, told the emergency session in Geneva.
Also addressing the meeting, Ms Pansieri said UN officials continued to gather “strong evidence” that serious violations of international human rights and humanitarian law had been committed in areas under IS control.