The banality of the evil of ISIS has been laid bare by a UN official who has described the way in which women and children are being sold. Zainab Bangura, United Nations (UN) Special Representative on Sexual Violence in Conflict says a price list for slaves is being circulated. It's reported to cost around £100 to buy an adolescent girl in the areas of Iraq and Syria which are controlled by ISIS.

"The girls get peddled like barrels of petrol," she said. "One girl can be sold and bought by five or six different men." She also suggested that there is a hierarchy of 'customers' for the slaves who are first offered to the leaders of ISIS, then to foreign buyers, then to ISIS fighters.

Christian Today has previously reported Bangura's dire warnings about the treatment of women and children at the hands of ISIS fighters. But now she has gone further and suggested that they may constitute war crimes. "These appalling crimes of sexual violence in conflict, which may amount to war crimes, crimes against humanity and/or genocide will not be forgotten."

Christians are among those who have been taken captive by the Islamic State along with Yazidis, Shi'as and others who don't conform to its strict interpretation of Sunni Islam.

ISIS documents have shown that income from slaves provides part of the funding for their expansion, but the authenticity of the so called 'price list' of slaves hadn't been confirmed. Bangura now says, "They have a machinery, they have a programme. They have a manual on how you treat these women. They have a marriage bureau which organizes all of these 'marriages' and the sale of women. They have a price list."

As Western leaders and those in the region itself continue to debate how to contain or defeat ISIS, Bangura said the group presents a unique problem, "It's not an ordinary rebel group. When you dismiss them as such, then you are using the tools you are used to. This is different. They have the combination of a conventional military and a well-run organized state."