The army personnel are pictured being led away and then lying in trenches before and after their "execution".
Iraqi military spokesman Lt Gen Qassim al-Moussawi said the pictures were authentic and depicted events in Salahuddin province.
But the images' authenticity has not been independently confirmed.
The BBC's Jim Muir, in northern Iraq, says if the photographs are genuine, it would be by far the biggest single atrocity since the time of the American-led invasion of Iraq in 2003.
Their emergence came as the Iraqi government claimed to have "regained the initiative" against the offensive by Sunni rebels led by ISIS - the Islamic State in Iraq and the Levant.
Extremists captured key cities, including Mosul and Tikrit, last week, but several towns have now been retaken from the rebels. » | Sunday, June 15, 2014
