DOHUK (Iraq) • Somewhere in territory controlled by the Islamic State in Iraq and Syria (ISIS), Ms Kayla Mueller, an American aid worker, was chained in a room with other female captives.

ISIS leader Abu Bakr al-Baghdadi would regularly enter and lead Ms Mueller away, her parents said.

"She would be gone for some time, taken to Baghdadi's room, and then she would come back crying, and tell the other girls what had happened," said her mother, Mrs Marsha Mueller, in a telephone interview last Friday. She was relaying the account of her daughter's rapes, which was shared with the family by United States officials.

The information, first published by British newspaper The Independent, confirms reports that Ms Mueller was a sex slave of ISIS, which has created a system of organised rapes of women it considered as infidels.

ISIS said she was killed in a Jordanian air strike on Feb 6. She would have been 26 at the time of her death. The group had sent her parents pictures as proof. But Jordanian and US officials have expressed doubt about ISIS' account of her death after 18 months as a hostage.

Details of her captivity, including her rapes by the leader of the organisation, were pieced together by US officials. They based the account on interviews with two girls from the Yazidi religious minority who were enslaved alongside Ms Mueller, her parents said.

"We were told that Kayla had to 'marry' Baghdadi, but we all understand what that means," said her father, Mr Carl Mueller. Ms Mueller had gone to Turkey in December 2012 to work for a Turkish organisation providing humanitarian aid to Syrian refugees along the Syrian border. She was abducted on Aug 4, 2013, in northern Syria.

Ms Mueller's parents said they first thought she had become the sex slave of a man known as Abu Sayyaf, a Tunisian ISIS figure killed in a US raid in May. Baghdadi personally took her to be imprisoned inside Sayyaf's Syrian home, US counter-terrorism officials told ABC News.

But in June, her parents said, US officials told them she had been Baghdadi's captive. The officials drew that conclusion partly by interviewing Sayyaf's wife and a Yazidi slave seized in a US special operations raid on Sayyaf's Syrian compound in May, when he was killed.

Mrs Mueller said her daughter was a protector of the younger Yazidi girls. "They looked at her as a mother figure," she said.

NEW YORK TIMES, REUTERS