Isis: Kayla Mueller was held as sex slave by IS leader Abu Bakr Baghdadi

American hostage Kayla Mueller was held as a personal sex slave by Isis leader Abu Bakr Baghdadi.

The Arizona aid worker was taken hostage in August 2013 in Aleppo, Syria, along with a number of Syrian aid workers, including her boyfriend, after leaving a Spanish Medicins Sans Frontieres hospital.

While the others were released IS demanded a $6.6m (£4.3m) ransom for Miss Mueller.

Miss Mueller was often reported to be kept in the company of an IS leader who had custody over her, according to an official quoted by ABC News who referred to indirect intelligence they managed to gather in Syria.

News that she was married to an IS fighter was first reported by CBS News.

In June, U.S. intelligence officials confirmed to her family that she had in fact been forced to "marry" Baghdadi, Sky News reports.

Speaking on Friday (14 August) on what would have been his daughter's 27th birthday, Kayla's father Carl Mueller told The Associated Press: "They told us that he married her, and we all understand what that means."

Her mother, Marsha Mueller, added: "Kayla did not marry this man. He took her to his room and he abused her and she came back crying."

Mueller was held captive by ISIS financier Abu Sayyaf and his wife, known as Umm Sayyaf. She was held with three Yazidi women including a teenage girl who escaped and recounted the ordeal of the women, including Kayla at the hands of Baghdadi.

She revealed that Al-Baghdadi took Mueller as a "wife," repeatedly raping her when he visited. In a state of distress, she would tell her fellow captives what had happened.

When the teenager planned to escape with her sister she asked Mueller to accompany her, the parents were told, but Kayla refused, worrying that her obvious Western appearance would lead to their capture.

"Kayla tried to protect these young girls," her mother said. "She was like a mother figure to them."

The 14-year-old escaped in October 2014 and fled to Iraqi Kurdistan, where she talked to U.S. commandos, revealing the circumstances of Kayla's captivity.

However, by the time the Yadizi escapee reported the situation to Delta Force commandos in Iraq, Kayla had been moved, her parents were told.

Abu Sayyaf was killed in a Delta Force raid of his Syrian compound in June. His wife Umm Sayyaf was captured and while under American interrogation in Iraq, she confirmed that al-Baghdadi had "owned" Kayla.

US president Barack Obama revealed in an interview with Buzzfeed news that Mueller was one of the hostages that US forces tried to rescue in a failed operation in Syria last July.

"I deployed an entire operation - at significant risk - to rescue not only her but the other individuals that had been held and probably missed them by a day or two," Obama said.

Several Isis-linked Twitter accounts have claimed that the aid worker was killed in the Jordan bombing raid on the Syrian de facto capital of IS, Raqqa.

Umm Sayyaf has been turned over to the Iraqi Kurds for trial. The Muellers have been told she can be expected to serve a long prison sentence.

Isis routinely hold women as sex slaves, often selling them in barbaric slave markets and passing them around between fighters.

A senior United Nations official has confirmed that Isis is circulating a sickening "sex slave price list" for captured women and children.

According to VICE News, Isis first began selling sex slaves after it captured over 2,000 Yazidis in August 2014. Channel 4 in the UK and PBS in the US recently released a documentary called Escape from Isis showing Yazidi sex slaves escaping the horrors of enslavement in Syria and Iraq.