On their rampage through Iraq, Sunni Islamist insurgents boast that they have slaughtered 1,700 government soldiers.

Neither that claim nor a set of gruesome photographs posted on Twitter can be authenticated.

But the terror of captives shown in the still pictures looks all too real. In civilian clothes, men are shown walking in a line, bent over and looking at their feet as a man points a gun at them. Another photo shows men loaded into a truck. Another features a group lying in a shallow grave as insurgents take aim. The final images show bodies soaked in blood.

The horror depicted seems to be confirmed by families desperately trying to escape the fighting. Speaking on condition of anonymity, one man who had fled the northern Iraqi town of Sulaiman Pek said he had seen dead soldiers – about 30 bodies in total – lying on the road.

He had no doubt about the jihadists’ ruthlessness.

“They said from the start that anyone who resists them will be killed and that anyone who surrenders his weapons can go.”

Thousands of people are seeking shelter in Kurdish-controlled areas, like Erbil.

Many of them are fleeing from Mosul, the militants’ biggest prize yet, and end up in camps for displaced people.