Sunni rebels in Iraq have posted pictures on Twitter apparently showing their fighters killing captured Shia soldiers.
A Twitter account aligned with the Islamic State of Iraq and the Levant (ISIL), said the pictures show scores of captured Iraqi soldiers being killed.
The images, released at the weekend, show masked fighters 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.
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.
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.
On Friday, UN human rights chief Navi Pillay warned against "murder of all kinds" and other war crimes in Iraq, saying the number killed in recent days may run into the hundreds.
She said in a statement that her office had received reports that fighters rounded up and killed Iraqi soldiers as well as 17 civilians in a single street in Mosul.
Her office also heard of "summary executions and extrajudicial killings" after ISIL and associated groups overran Iraqi cities and towns.
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On Monday, rebels captured the strategic town of Tal Afar, close to the Syrian border, after a long and bloody offensive against Iraqi forces.
Sources in the town told Al Jazeera Sunni rebels were roaming the streets after the Iraqi military was purged.
Al Jazeera's Hoda Abdel-Hamid, reporting from the northern town of Khazer, said Tal Afar represented one of the last strongholds for the Baghdad government.
The town has provided a crucial bulwark against rebel-controlled territory on either side of Iraq's border with Syria.
The fall of Tal Afar comes a week after rebels captured Iraq's second-largest city, Mosul, and Saddam Hussein's hometown of Tikrit in a lightening offensive.