Amid all the atrocities carried out by Isis — its massacres of civilians, its beheading of hostages, its pillaging of antiquities — the systematic violence the jihadists have carried out against countless enslaved women and girls never fails to shock. For months now, we've heard appalling testimony from women who escaped Isis's clutches, many of whom endured rape and other hideous acts of violence.

Zainab Bangura, the U.N.'s special representative on sexual violence in conflict, recently conducted a tour of refugee camps in the shadow of the conflicts in Syria and Iraq, war-ravaged countries where the Islamic State commands swaths of territory. She heard a host of horror stories from victims and their families and recounted them in an interview earlier this week with the Middle East Eye, an independent regional news site.

"They are institutionalizing sexual violence," Bangura said of the Islamic State. "The brutalization of women and girls is central to their ideology."

Bangura detailed the processes by which "pretty virgins" captured by the jihadists were bought and sold at auctions.

Isis have destroyed homes and infrastructure in northern Iraq and Syria (Getty)

Here's a chilling excerpt: "After attacking a village, [the Islamic State] splits women from men and executes boys and men aged 14 and over. The women and mothers are separated; girls are stripped naked, tested for virginity and examined for breast size and prettiness. The youngest, and those considered the prettiest virgins fetch higher prices and are sent to Raqqa, the IS stronghold.

There is a hierarchy: sheikhs get first choice, then emirs, then fighters. They often take three or four girls each and keep them for a month or so, until they grow tired of a girl, when she goes back to market. At slave auctions, buyers haggle fiercely, driving down prices by disparaging girls as flat-chested or unattractive.

We heard about one girl who was traded 22 times, and another, who had escaped, told us that the sheikh who had captured her wrote his name on the back of her hand to show that she was his 'property'."

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Estimates vary, but there are believed to be somewhere between 3,000 and 5,000 women enslaved by the Islamic State. Many are Yazidis, a persecuted minority sect that the extremist Islamic State considers to be apostate "devil-worshippers," in part because of the Yazidis' ancient connection to the region's pre-Islamic past. The jihadists' treatment of Yazidi women, in particular, has been marked out by its contempt and savagery.

Here's Bangura again: "They commit rape, sexual slavery, forced prostitution and other acts of extreme brutality. We heard one case of a 20-year-old girl who was burned alive because she refused to perform an extreme sex act. We learned of many other sadistic sexual acts. We struggled to understand the mentality of people who commit such crimes."

Hundreds of Yazidi women and girls have escaped their captors, either by running away, or being ransomed and rescued by their families. Bangura has urged international assistance in providing proper medical and "psychosocial" support to the escaped women, who have experienced terrible trauma.

"It was painful for me. The countries I have worked on include Bosnia, Congo, South Sudan, Somalia and Central African Republic," says Bangura, a former former minister of Sierra Leone who is no stranger to conflicts. "I never saw anything like this. I cannot understand such inhumanity. I was sick, I couldn’t understand."

In a separate interview with the Toronto Star, Bangura warned that neither U.N. agencies nor regional authorities are currently able to provide the sort of extensive care many of the escaped captives may need. She said the resilience and ability of these women to build back their lives would help "strip victory away" from the militants.

"This is precisely what [the Islamic State] does not want," says Bangura. "It can be a kind of vengeance, helping these women recover and giving them a path to thrive."