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 UN allegations came as a BBC team entered the town of Amerli, in northern Iraq, where residents and fighters are celebrating the end of a two-month siege and the withdrawal of IS militants from around the town.
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.
Ms Pansieri said Christian, Yazidi, Turkmen, Shabak, Kaka'i, Sabeans and Shia communities had "all been targeted through particularly brutal persecution" and that IS had "ruthlessly carried out what may amount to ethnic and religious cleansing".
"Many have been killed directly; others have been besieged and deprived of food, water or medication.
"Hundreds of thousands of civilians from these communities have fled to remote and desolate locations where unconfirmed reports indicate that scores of children, elderly people and people with disabilities have been dying as a result of exhaustion and deprivation."
Yazidis have been targeted for extremely harsh treatment. Many men who refused to convert to Islam were reportedly executed, while women and young girls were allotted as slaves to IS fighters. At least 2,250 Yazidi women and children are reportedly being held hostage.
Ms Pansieri said evidence suggested that Iraqi government forces had also killed detainees and shelled civilian areas.
Last week, the UN said it had received reports of at least 650 male inmates of Badouch Prison in Mosul being shot dead by IS militants on 10 July. Witnesses and survivors said inmates claiming to be Sunni were taken away, while Shia and members of other religious or ethnic communities were ordered into ditches and killed.
Ms Pansieri also said Iraqi security forces personnel and anti-IS militia might have perpetrated violations of human rights and humanitarian law.
On 22 August, members of "volunteers units" affiliated with the government and other armed men were said to have killed 73 men and boys worshipping at a Sunni mosque in Bani Wais, Diyala province.
Shelling and air strikes by Iraqi security forces have also killed and injured many civilians. On 14 and 15 August, two air strikes in the Hawija area of Kirkuk left 25 people dead, while in Falluja 17 civilians were killed by shelling between 14 and 17 August.
Ms Pansieri concluded by urging the international community to intensify its efforts to protect all Iraqis and ensure that any individuals who had participated in, or supported, crimes against humanity and war crimes were held accountable.
In a separate development on Monday, Iraqi officials told the AFP news agency that Kurdish peshmerga fighters and Shia militiamen had retaken Suleiman Bek, a key stronghold for IS over the past 11 weeks.