Slavery, rape, forced prostitution and fiery death are among the fates that await women and girls abducted by ISIS, the United Nations's special representative on sexual violence in conflict says.

"The countries I have worked on include Bosnia, Congo, South Sudan, Somalia and Central African Republic; I never saw anything like this. I cannot understand such inhumanity. I was sick," Zainab Bangura told the Middle East Eye, an independent regional news site, after investigating ISIS sex crimes in Iraq, Syria, Turkey, Jordan and Lebanon.

"They are institutionalising sexual violence; the brutalization of women and girls is central to their ideology," she said.

"They use sexual violence as a 'tactic of terrorism' to advance key strategic priorities, such as recruitment, fundraising, to enforce discipline and order — through the punishment of dissenters or family members — and to advance their radical ideology. They commit rape, sexual slavery, forced prostitution and other acts of extreme brutality."

What happens when ISIS conquers a village?

ISIS splits the men from the women and executes boys and men over 14, Bangura says. Girls are taken from their mothers, stripped naked, tested for virginity and examined for breast size and attractiveness.

Young, pretty virgins are sent to the ISIS stronghold and Raqqa, where they serve as sex slaves for militants.

"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."

The girls who don't meet ISIS's standards are sold at auction, as are the girls militants later grow tired of.

Do they ever escape?

Sometimes they get out with the help of their families, or even their captors' empathetic wives.

"We heard few stories of wives who helped the slaves to escape," Bangura said. "Some are released when a ransom is paid."

Others try a more extreme method of escape.

"When IS discovered girls used their headscarves to hang themselves, they forced them to remove them. I learned of three girls who tried to commit suicide by drinking rat poison, which had been left in a room. They started vomiting and were rushed to hospital and washed out. When they came back, they were brutally attacked."

What happens to the survivors?

Bangura met with ISIS escapees in Iraq's Kurdish zone, where officials are already dealing with a massive influx of refugees from ISIS and Iraq.

"I met one woman who was in shock -- most of her family had either been taken or killed. She was looking after her four-year-old son and trying to track down her 15-year-old daughter, who was taken by IS. She was so traumatized that she insisted her husband was missing, although he was dead," Bangura said. "Women like her need qualified medical and psycho-social support that is not readily available."