Forces allied with the Islamic State of Iraq and the Levant (ISIL) have almost certainly committed war crimes by executing hundreds of non-combatant men over the past five days, UN human rights chief Navi Pillay said.
Monday's statement comes after reports showed that soldiers, military conscripts, police and others who had surrendered or been captured had been summarily executed.
"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."
The group has also executed 13 imams in Mosul for refusing to pledge allegiance to ISIL, she said.
Previously, Sunni rebels in Iraq posted pictures on Twitter apparently showing their fighters killing many Shia soldiers.
The pictures, posted over the weekend, purport to show scores of captured Iraqi soldiers being killed by ISIL, who wrested control of cities and towns in northern Iraq last week.
The images show masked fighters loading the captives onto trucks before forcing them to lie face-down in a shallow ditch with their arms tied behind their backs. The final pictures show the bodies of the captives soaked in blood after being shot.
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A caption on one of the pictures suggested that the victims - believed to be security personnel - were punished for fighting Sunnis. The Twitter account where the pictures were posted was suspended hours later.
The ISIL and associated Sunni rebel forces have taken control of large territories in Iraq in the last week, plunging the country into crisis and increasingly splitting communities along sectarian lines.
A diplomatic source in Baghdad told Al Jazeera that the pictures were authentic but the number of soldiers executed could not be verified.
Chief military spokesman Lieutenant-General Qassim al-Moussawi also confirmed the photos' authenticity and said he was aware of cases of mass murder of captured Iraqi soldiers in areas held by ISIL.
He told Associated Press that an examination of the images by military experts showed that about 170 soldiers were shot to death.
Captions on the photos showing the soldiers after they were shot say "hundreds have been liquidated".
US State Department spokeswoman Jen Psaki said that the ISIL fighters' claim of killing the Iraqi troops "is horrifying and a true depiction of the bloodlust that those terrorists represent".
She added that a claim that 1,700 were killed could not be confirmed by the US.
Source: Al Jazeera and agencies