Disturbing images have been distributed by the Sunni extremists who have taken territory across Iraq.
According to the group several photos show dozens of captured Iraqi soldiers being executed.
If proved to be genuine, the systematic killings would be the biggest single atrocity since the fall of Sadam Hussein in 2003.
The insurgents from ISIL or the Islamic State of Iraq and the Levant are said to have now seized the city of Tal Afar near the Syrian border.
It follows the fall of Mosul, Iraq’s second biggest city and Tikrit. Thousands of Iraqis have fled but concern is growing for those trapped by the fighting.
After sweeping through towns towards Baghdad, ISIL fighters appear to have halted their advance on the capital, instead moving to tighten their grip in the north.
On Sunday Iraqi Prime Minister Nuri al-Maliki gave a speech to volunteer fighters at a military camp in Baghdad after thousands of Shiites across southern Iraq answered an urgent call to arms to help fight the Islamic militants.
But at one recruitment centre in Khalis – one of the last big towns in government hands north of the capital – six people were killed when volunteers came under mortar attack.
On the international front the US is reviewing military options short of sending in troops and is also preparing to talk with its longtime adversary, Iran on how to push back the extremists.