Islamic State terrorists have revived the primitive practice of sex slavery and are selling captured girls for as little as the price of a packet of cigarettes.

The girls, some extremely young, are then taken to territories they control and used as bait to lure recruits, including foreign fighters.

The latest reports follow earlier allegations of children being murdered, and of ISIS propaganda advocating the rape of children.

Record numbers are signing up to fight with ISIS. Only last week, Jurgen Stock, who heads Interpol, said that more than 4,000 foreigners are with jihadists in Syria and Iraq, a five-fold increase in under a year.

Latest data from the UN indicates many of these are being attracted by the sex for sale in ISIS controlled territories.

Zainab Bangura, UN envoy on sexual violence, told AFP in an interview this week that ISIS is selling girls in Syria and Iraq sometimes "for as little as a pack of cigarettes."

Speaking on her return from a tour of Europe to highlight what is being done to women, she said: "They kidnap and abduct women when they take areas so they have – I don't want to call it a fresh supply – but they have new girls. This is a war that is being fought on the bodies of women."

Describing those who had escaped and were now living in refugee camps in Turkey, Lebanon and Jordan, she continued: "Some were taken, locked up in a room – over 100 of them in a small house – stripped naked and washed." They were then forced to stand in front of men so they could asses "what they are worth."

She said it was part of the recruitment strategy. "This is how they attract young men – we have women waiting for you, virgins that you can marry. The foreign fighters are the backbone of the fighting."

She said the practices were "medieval". She said IS wants to build "a society that reflects the 13th century."

According to a recent UN report, as many as 25,000 foreign fighters from more than 100 countries were involved in conflicts worldwide, with the largest influx by far into Syria and Iraq.