The Sunni extremist group that has taken territory across Iraq has posted photos online which appear to show its fighters massacring Iraqi soldiers, Radio New Zealand reports.
The army personnel are pictured being led away and then lying in trenches before and after their "execution".
The BBC reported Iraqi military spokesman Lt Gen Qassim al-Moussawi as saying the pictures were authentic and depicted events in Salahuddin province.
But the images' authenticity has not been independently confirmed.
A BBC correpondent in northern Iraq said if the photographs were genuine, they would depict by far the biggest single atrocity since the time of the American-led invasion of Iraq in 2003.
The emergence of the pictures came as the Iraqi government claimed to have "regained the initiative" against the offensive by Sunni rebels led by ISIS - the Islamic State of Iraq and Syria.
Extremists captured key cities, including Mosul and Tikrit, last week, but several towns have now been retaken from the rebels.
The Iraqi government said its forces have killed 279 militants in 24 hours, as they pushed back against the offensive.
The announcement from the prime minister Nuri al-Maliki's security spokesman came after the United States secretary of state John Kerry said US assistance to Iraq would only work if Iraqi leaders overcame deep divisions.
Kerry spoke with Iraq's foreign minister about providing assistance in fighting ISIS.