The United Nations' top commissioner for human rights argued Sunday that the Islamic State was committing "crimes against humanity" and offered proof of a prison massacre in which ISIS militants separated the inmates into Sunni and Shia groups and killed all the Shias.

That report confirms what had long been suspected after images of alleged mass executions emerged on ISIS killers' social media accounts: Nearly 700 Shias were killed by armed men from the Islamic State of Iraq and al-Sham in systematic, concentration-camp style at a prison in Mosul, Iraq, two months ago, UN High Commissioner for Human Rights Navi Pillay told the Guardian:

Citing testimony from eyewitnesses and survivors, the UN said Islamic State gunmen arrived at the front gate in a group of pick-up trucks. Several carried machine guns. They took out the prisoners and sorted them into two groups, Sunni and 670 Shias. The fighters grilled the Sunni group, asked them to recite prayers, and interrogated them about family backgrounds. Some Shia prisoners tried to pass themselves off as Sunni. They were discovered and returned to the Shia line-up.

The Islamic State militants told their Shia captives they would be "released" once their identities were verified. The prisoners had to give a number in turn – beginning with one, with the last prisoner saying: "I'm 679". The fighters then loaded the them into trucks and drove three to four kilometres south-east to an uninhabited "desert-like location", somewhere between Mosul's main road and its railway line.

According to the UN, the prisoners were lined up in four rows. They were told to kneel, and then shot.

Incredibly, 20 men survived the massacre by playing dead and were later interviewed, along with other eyewitnesses, about the Islamic State's brutal activities. "Such cold-blooded, systematic and intentional killings of civilians, after singling them out for their religious affiliation may amount to war crimes and crimes against humanity," Pillay told the Guardian.

In other ISIS news, reports from pan-Arab media sources say the Islamic State last week also announced the beheading several of its own leaders suspected of working with foreign governments. Among those allegedly executed was Abu Obaidah al-Maghribi, a Syria-based intelligence officer for the Islamic State, who'd been accused of "reporting information on the activities, movements, and plans of ISIS to the United Kingdom."

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