The UN's human rights chief has said Sunni fighters in northern Iraq have "almost certainly" committed war crimes, following the publication of photos and videos of the purported execution of scores of captives.

Navi Pillay said on Monday that corroborated reports showed that soldiers, military conscripts, police and others who had surrendered or had been captured had been summarily executed by the Islamic State of Iraq and the Levant and its allies.

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"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," she said.

Her comments came after a Twitter account sympathetic to the ISIL posted pictures apparently showing their fighters killing captured Shia soldiers. 

Masked fighters are shown loading the captives on trucks before forcing them to lie face-down in a shallow ditch with their arms tied behind their backs. The final pictures show bodies soaked in blood after being shot.

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.

In a separate post on Youtube, fighters are seen goading captives in Iraqi army uniform to say that the ISIL will live forever, before one of the captives is shot in the head.

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

Who are the Islamic State of Iraq and the Levant?

A diplomatic source in Baghdad told Al Jazeera that the pictures were authentic but the number of soldiers executed could not be verified.

Qassim al-Moussawi, Iraq's cheif military spokesman, said the photos were authentic and said he was aware of cases of mass murder of captured Iraqi soldiers in areas held by ISIL.

He told the AP news agency that an examination of the images showed that about 170 soldiers were killed.

Captions on the photos showing the soldiers after they were shot say "hundreds have been liquidated".

Ban Ki-moon, the UN secretary general, called the attack "deeply disturbing", and called on Iraqi leaders to ensure their followers avoided acts of reprisal.

Source: Al Jazeera and agencies