UN human rights chief Navi Pillay on Monday condemned a series of "cold-blooded" killings of Iraqi soldiers and religious leaders by Sunni militants, saying it amounted to war crimes.
"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."
Militants from the Islamic State of Iraq and the Levant (ISIL) jihadist group have seized the north Iraqi cities of Mosul and Tikrit and have vowed to move on Baghdad to topple the Shiite-dominated government.
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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.
"The provocative language used by ISIL, which has been talking about 'liquidating herds of sheep' and inciting sectarian tensions is clearly intended to sow further chaos and bloodshed in the country," Pillay 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.