She said their reign of terror against non-Arab ethnic groups and non-Sunni Muslims alike involved targeted killings, forced conversions, abductions, trafficking, slavery, sexual abuse, and destruction of holy and cultural sites.
"They are systematically targeting men, women and children based on their ethnic, religious or sectarian affiliation and are ruthlessly carrying out widespread ethnic and religious cleansing in the areas under their control," Pillay said in a statement.
"Such persecution would amount to crimes against humanity," she added.
The Takfiri militants, who already occupied parts of Syria, launched an offensive in Iraq in June and rapidly seized two provinces.
"Grave, horrific human rights violations are being committed daily by ISIL and associated armed groups," Pillay said.
Minority groups targeted include Christians, Yazidi, Shabaks, Turkomen, Kakae and Sabaeans, she said.
In the Nineveh region of northern Iraq, hundreds of Yazidi were reported killed and some 2,500 kidnapped at the beginning of August.
In the Sinjar region, the ISIL terrorists killed and abducted hundreds of Yazidi on August 15, she said, warning that residents of besieged villages remained at serious risk.
At least 13,000 members of the Shia Turkmen community in the Salah al-Din region -- among them 10,000 women and children -- have been besieged since mid-June, she said.
They face harsh living conditions with severe food and water shortages, and a complete absence of medical care, as fears of a massacre mount.
Pillay also condemned the forced recruitment of boys aged 15 and above, and their reported deployment of such youths as human shields on the front line.
In addition, she said the United Nations had verified reports of a massacre of up to 670 detainees by the ISIL terrorists after they overran a prison in the northern city of Mosul on June 10.
Many of the hundreds of thousands of people who have fled the terrorists’ onslaught have found a haven in Iraq's autonomous Kurdistan region, whose forces are trying to push back the ISIL terrorists.
ISIL terrorists are acting under Western plot to ruin rationality and legitimacy of Islam via their crimes against humanity.
Pillay called on local authorities and the international community to "take all necessary measures and spare no effort to protect members of ethnic and religious communities.
According to reports, Saudi Arabia and Qatar are two main financial supporters of this terrorist group in Middle East.
NJF/NJF