The Sunni extremist group that has taken territory across Iraq has posted photos online that appear to show its fighters massacring Iraqi soldiers.
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.
‘Taken to their deaths’
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.
Video footage, apparently filmed earlier, shows many hundreds of men being marched off from near the Speicher base, with the voice on tape saying they were soldiers who had surrendered at the facility.
Still photographs show large numbers of young men being transported away in trucks.
Captions on the photographs, which again appeared to have been issued by ISIS, said they were being taken away to their deaths.
What appear to be ISIS fighters are seen firing on their captives in some images.
Sources in the rebel movement, who do not belong to ISIS, told the BBC they believed as many as 1,000 personnel from the Speicher base had been executed.
They said the ISIS fighters had divided them into regular army conscripts, who were set free, and volunteers from Shia militias or from the government’s elite Golden Brigade, who were condemned to die.
Fightback
Earlier, Iraqi army spokesman Lt Gen Qasim Ata, said the military had scored successes against the militants in several areas, killing 279 of them. The figure could not be independently verified.
There were reports of heavy clashes round the city of Tal Afar, west of Mosul, with mortar shelling of some districts as militants tried to enter the city.
Government forces were reported to be building up in the city of Samarra, north of Baghdad, ready for a counter-offensive on Tikrit.
Washington remains concerned at the security situation in Iraq and has announced it is increasing security at its embassy in Baghdad but relocating some staff to safer areas.
The embassy would remain “fully equipped to carry out its national security mission”, the state department said.
The USS George HW Bush aircraft carrier is being deployed to the Gulf, accompanied by two more warships. But the US says no US troops will be deployed on the ground in Iraq.
Neighbouring Iran has offered direct assistance to Baghdad, but denies sending troops. However, there are reports of more than 130 Revolutionary Guards on hand to provide training and advice.
Meanwhile, former UK Prime Minister Tony Blair has insisted that the 2003 Anglo-US invasion of Iraq is not to blame for the violent insurgency now gripping the country.
Speaking to the BBC, he said there would still be a “major problem” in the country even without the ousting of Saddam Hussein.