A Yazidi woman who was held captive and kept as Isis leader Abu Bakr al-Baghdadi's personal sex slave has revealed the harrowing details of the detention of US hostage Kayla Mueller. The 26-year-old aid worker was kept as al-Baghdadi's secret bride before being killed in an air strike.

Sixteen-year-old Muna revealed that Isis militants ripped out Mueller's finger nails before she was repeatedly raped by Baghdadi. The American hostage was told by the terrorist leader that she would become his wife "by force," the Yazidi teen explained.

According to the Daily Mail, the notorious terrorist may have been the leader of one of the biggest terrorist organisations, but he was not above being afraid of his wives finding out about his American "bride". Muna told the paper that Baghdadi insisted Mueller be kept prisoner at the home of his deputy Abu Sayaff, where she would ultimately die.

The blue-eyed Yazidi spoke of her time in captivity and revealed that she was chosen by Baghdadi from a group of 61 Yazidi women and girls between the ages of nine and 22 in Mosul. After being held for two nights by five militants in the Islamic State's so-called capital of Raqqa, Muna said she was picked by Baghdadi on 15 August 2014. He reportedly took Muna to live in a two-storey home in the centre of Raqqa along with his three wives and children.

During her imprisonment, Muna said she was forced to watch the execution video of American journalist James Foley by the British jihadi Mohammed Emwazi, or Jihadi John. "He said he ordered them to kill James Foley," Muna said.

Muna was told she was an infidel and would only be allowed into heaven if she converted to the "nice and clean" religion of Islam, the Daily Mail reported. The girl would attempt to escape after 10 days in Raqqa, but she was unfortunately recaptured by Isis and brought back to Baghdadi.

Muna suffered horrific torture by Baghdadi, which included beatings with a belt, a garden hoe, cables and wooden sticks. Baghdadi once beat the girl so severely that he left her with a dislocated shoulder, she said. She was later transferred to a female prison in Raqqa, where she met Mueller.

Mueller was kidnapped by Isis fighters in August 2013, along with her Syrian photographer boyfriend Omar Alkhani. Alkhani allegedly attempted to plead for her release by telling their captors she was his wife. However, when she told the fighters the truth, she was claimed by Baghdadi. Alkhani was later released.

"When they first captured Kayla, Isis fighters pulled her nails out," Muna said. "She was forced to convert to Islam." Muna said she, Mueller and another Yazidi sex slave named Susan were transported to the home of Sayaff in the Al-Shadadiya suburb of Raqqa. By October, Baghdadi visited the home and reportedly raped the American hostage repeatedly.

"She was afraid and she didn't resist as he would beat her," Muna said. "I saw him rape her three or four times and they told us that anybody who runs away, including Kayla, would be killed...Kayla did not know he was the Caliph, but she knew he was important."

Baghdadi reportedly forced Mueller to wear a hijab full body covering and to learn about the Quran. Mueller would go on to reject Muna and Susan's plan to escape, Muna explained. "Susan and I insisted on running away and told Kayla to come with us. But she said her friend [James Foley] had already been beheaded and she feared suffering the same fate," she said.