"Based on corroborated reports from a number sources, it appears that hundreds of non-combatant men were summarily executed over the past five days, including surrendered and captured soldiers, military conscripts, police and others associated with the (Baghdad) government," Pillay said in a statement.
"Although the numbers cannot be verified yet, this apparently systematic series of cold-blooded executions, mostly conducted in various locations in the Tikrit area, almost certainly amounts to war crimes."
Photos posted online reportedly show them executing scores of Iraqi soldiers as they pushed their advance on the capital in a "horrifying" massacre that has drawn international condemnation.
But their victims are also said to have been religious leaders, including the imam of the Grand Mosque in Mosul and 12 local imams for "refusing to pledge allegiance to ISIL," the statement from the UN High Commissioner for Human Rights said.
She warned the new wave of fighting in Iraq was a threat to the entire region, and called on Iraqi political and religious leaders to unite against "these efforts to rip the country apart along sectarian or geographic lines".
As Iraqi forces brace to defend Baghdad, the United States -- whose 2003 invasion toppled Sunni strongman Saddam Hussein -- and Shiite-majority Iran have raised the possibility of working together to help resolve the crisis.