GETTY•REUTERS ISIS leader Abu Bakr al-Baghdadi and US hostage Kayla Mueller

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US hostage Kayla Mueller was tortured and kept imprisoned by Abu Bakr al-Baghdadi after her capture in August 2013 during a brief trip to a hospital in ISIS-held territory in Syria. She was forced to convert to Islam and after admitting she was not married, ISIS high command set about finding her a husband before al-Baghdadi claimed her for himself. The 26-year-old was told she would become the vile jihadi's wife "by force" and was kept imprisoned in the home of al-Baghdadi's deputy, Abu Sayaff.

There was no formal wedding ceremony but she was forced to wear a hijab covering her entire body and he spent days on ends teaching her about the Koran. Harrowing details of the final weeks Ms Mueller spent alive reveal she was sexually and physically abused before she was killed by either her depraved captors or shortly afterwards in a coalition airstrike. The charity worker tragically missed out on being rescued by US special forces by just a day or two. Although she was kept in the home of Sayyaf, a senior ISIS leader, along with several other sex slaves, al-Baghdadi would routinely visit the group to rape and sexually abuse them.

GETTY al-Baghdadi is very rarely seen in Public

Sayyaf was banned from even looking at his houseguest and al-Baghdadi grew jealous about his newly-acquired bride. The ISIS deputy was killed in a Delta Force raid on his Syrian compound in June, which resulted in a treasure trove of intelligence about ISIS. The revelation about Ms Mueller's final months emerged from a Yazidi girl named only as Muna, who was also kept as a sex slave by al-Baghdadi and met the American on several occasions. The 16-year-old also spoke out about her experience at the hands of the ISIS leader, describing systematic torture and rape.

She said she was handpicked from a group of 61 girls at a local slave market – where some of those were aged as young as nine – before being transported from location to location at the whim of al-Baghdadi. The blue-eyed youngster was held for two nights under the guard of five militants in Raqqa before the ISIS leader arrived along with his son in law and bodyguard Mansour, a 30-year-old Kurdish man. She then spent time living with al-Baghdadi and his three wives elsewhere in the Syrian city.

The fight against ISIS Fri, November 18, 2016 The battle against ISIS militants (also abbreviated as Daesh, ISIL, IS and Islamic State) continues in the Middle East. Play slideshow Getty 1 of 183 Forces battle against ISIS

Describing the dysfunctional family set-up, she told MailOnline: “The wives were always worse than al-Baghdadi. “They were always telling the children that they were lazy and beating them.”

AP Ms Mueller was 26 when she was captured and killed

She was such as close friend to me - more than a sister. I loved Kayla Muna

She revealed how the ISIS leader and his family liked expensive watches and would eat takeaways rather than cooking or going out to restaurants. Muna first met Ms Mueller at a female-only prison in Raqqa. Muna eventually managed to escape from ISIS and made it to a safe house where she was rescued by Kurdish fighters. She now plans to make the journey to Germany, as thousands of other Syrian refugees have done.

Speaking of the moment she learned of the US aid worker's death, Muna said: "I was so sad, but I don't believe she was killed in an airstrike.

AP The aid worker was visiting a hospital in Syria when she was kidnapped