Islamic State has issued guidelines to its fighters saying that raping a woman ten times will convert her to Islam, a former sex slave has claimed.

Noor, a 22-year-old Yazidi woman, was abducted by ISIS when her village in the Sinjar province of Iraq was attacked by jihadists last year. She told CNN that she was bought by a militant who forced her to marry him, and later showed her a letter he'd received on the front line.

"This shows any captured women will become Muslim if 10 ISIS fighters rape her," the militant told her. Noor says she was then raped by her "husband", before he gave her to 11 of his friends who each raped her as well.

A minority ethno-religious group, Yazidis have been systematically persecuted by Islamic State during its attempts to create a caliphate across Iraq and Syria. Their religion is an offshoot of Zoroastrianism, which blends ancient religious traditions with both Christianity and Islam. ISIS believes them to be "devil-worshippers".

Around 3,000 women and girls, the majority of whom are Yazidis, are believed to remain in ISIS captivity, and the UN earlier this year said that militants may have committed genocide against the minority.

Noor was interviewed alongside two other young women who had managed to escape ISIS. Bushra, 21, said that militants forced captured women to have an abortion if they were found to be already pregnant.

A friend of hers was three months pregnant when she was kidnapped by ISIS. "They took her into another room. There were two doctors and they did the abortion," Bushra said. When her friend returned, she was bleeding heavily and in excruciating pain.

"I asked her what happened and how they did it," Bushra added. "She said the doctors told her not to speak."

Other escaped Yazidis have spoken of the systematic rape of women and children captured by ISIS. One woman told UN investigators that an Islamic State 'doctor' repeatedly raped her, and sat on her stomach telling her: "this baby should die because it is an infidel; I can make a Muslim baby."

ISIS last year issued guidelines specifying how its followers are to treat their female slaves.

Dated October/November 2014, the Q&A leaflet details exactly how they may buy, sell and abuse "unbelieving" women – thought to mean all those who are not Sunni Muslims.

It clarifies that "It is permissible to have intercourse with the female slave who hasn't reached puberty if she is fit for intercourse," and any female slave who attempts to run away is to be "reprimanded in a way that deters others like her from escaping".