The United Nations Human Rights chief, Navi Pillay, has condemned what she said were appalling and widespread crimes being committed by Islamic State forces in Iraq.
Ms Pillay, whose six-year term ends in the coming days, said the crimes included mass executions of prisoners and "ethnic and religious cleansing".
In a statement, she said the persecution of entire communities and systematic violations by the al-Qaeda offshoot, documented by UN human rights investigators, would amount to crimes against humanity and war crimes under international law.
Islamic State insurgents have captured a third of Iraq and declared a caliphate in the areas of Iraq and Syria it controls.
It has drawn the first American airstrikes in Iraq since the end of the occupation in 2011.
Meanwhile, Iraq's Prime Minister-designate Haider al-Abadi has said a "clear vision" on a new government will emerge within the next two days.
His comments come as the country faces a deepening sectarian conflict.
Mr Abadi is tasked with forming a power-sharing administration that can ease tensions and counter Islamic State militants.
He said the latest talks on the structure of the government had been constructive.
Shortly after Mr Abadi spoke, a suicide bomb attack in a Shia mosque in Baghdad today, killed at least nine people and wounded 21.
The attacker detonated his suicide bomb vest inside the mosque in the New Baghdad district of the capital at prayer time, police said.
In his comments today, Mr Abadi also emphasised that the central government will not tolerate armed groups operating outside government control.
"There is no place for armed groups whether from the militias, the tribes or the volunteers," he said.
"We will not allow the formation of armed groups out of control of the state."
The Prime Minister-designate noted that arms given to the peshmerga, the Kurdish force battling Islamic State militants in the north of the country, had passed through the central government.
Still, many ordinary Iraqis have expressed concern about the unchecked growth of violent militia groups in recent weeks, some of whom have been accused of targeting civilians.
In June, Ayatollah Ali Sistani, the most senior Shia cleric in the country, issued a call to arms for Iraqis to defend against the onslaught of the Islamic State as the group's fighters blazed across northern Iraq.
As a result thousands of volunteers, many of them Shia, joined militia groups which are only nominally under the control of the central government.
An attack on a Sunni mosque in Diyala province north of Baghdad on Friday killed 68 and wounded dozens.
Shia militants operating in the area are suspected of carrying out the attack, according to police sources and residents.
The central government has issued arrest warrants for four local tribesmen suspected of carrying out the attack after an investigation, Mr Abadi said today.