As rebel forces advanced towards Baghdad, reports began to emerge of summary mass executions carried out in territory they controlled.
Videos were posted purporting to show some of the killings and in one Twitter account, believed to be controlled by the rebel forces, it was claimed that 1,700 Shia soldiers had been executed.
The United Nations had already expressed alarm at reports of soldiers, police and civilians being rounded up and killed in areas taken over by Isis.
Navi Pillay, the UN High Commissioner for Human Rights, feared “murder of all kinds” and was looking into reports that Iraqi soldiers and 17 civilians had been summarily executed in a street in Mosul. “There will be particular scrutiny of the conduct of Isis, given their well-documented record of committing grave international crimes in Syria,” she said.
Among the images posted by Isis was a series of a police major who had his head hacked off after opening his door to armed men.