Militants from the radical Islamic group that controls parts of north and western Iraq have posted photos online that appear to show massacres of hundreds of captured Iraqi soldiers.
Pictures on a militant website allegedly show Islamic State of Iraq and the Levant (ISIL) fighters forcing captives to lie face-down in a ditch with their arms tied.
The last pictures show the bodies of the men soaked in blood after apparently being shot.
The killings were said to be revenge for the killing of an ISIL commander.
ISIL, an Al-Qaeda splinter group, captured Mosul and former dictator Saddam Hussein's hometown of Tikrit, last week.
Iraqi military spokesman Lieutenant General Qassim al-Mussawi said he believes the photos are authentic and said there have been reports of mass murders by soldiers.
UN human rights chief Navi Pillay said her office has heard of "summary executions and extrajudicial killings" by ISIL militants.
Based on reporting by AP, AFP, "The New York Times"