According to BBC News, the Sunni extremist group that has taken territory across Iraq has posted photos online that appear to show its fighters massacring Iraqi soldiers.
The pictures, apparently posted by ISIS, are said to show what happened to soldiers after the group took over an army base in Tikrit following the surrender of the garrison there, but it appears that all the people wear civilian not military clothing, ISIS claim they killed 1700 Shi'a men in Salahadin province.
Iraqi military spokesman Lt Gen Qassim al-Moussawi said the pictures were authentic and depicted events in Salahuddin province.
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.