The American aid worker Kayla Mueller was killed by ISIS, not a coalition airstrike, it has been claimed today.

Mueller, a Christian aid worker, 26, was abducted while leaving a hospital in Northern Syria in August 2013.

She was held captive for 18 months and raped by ISIS leader Abu Bakr al-Baghdadi, before being killed in what was believed to be a coalition airstrike in February 2015.

However, three Yazidi girls who were held alongside Mueller and have since escaped told the BBC that she was murdered by ISIS.

One of the three, Amshe, was a sex slave for Haji Mutazz – ISIS' second in command. She said Muttazz mocked the western reports blaming her death on the airstrike.

"Her stupid government," he reportedly said. "We killed her.

"We killed her because she is an American. If Obama wanted to he could tell Bashar [al-Assad, the Syrian president] to stop bombing us. Ten of our women and children die every day. This is our revenge. We will kill any American citizen we capture. If any Western government refuses to put pressure on Bashar, we will kill their citizens too."

According to Amshe, Muttazz said Mueller was considered a trophy due to her pale skin and American nationality.

ISIS claimed in early February that she was killed when Jordanian fighter jets bombed a building where she was being held outside Raqqa, an ISIS stronghold in Syria. Jordanian and US officials have expressed doubt about Islamic State's account of her death.

A report released by US authorities revealed that Mueller had been repeatedly raped by al-Baghdadi, which was confirmed by two other Yazidi girls who spoke to the BBC.

Dalal and Susan, both captured and sold into slavery by ISIS, shared a cell with Mueller. Dalal was 17 when she was abducted and Susan 13.

They said Mueller told them IS pulled her fingernails out in an attempt to make her admit she was a spy.

Despite being forced to pray and read the Koran out of fear, Dalal said Mueller never truly converted.

"She described her 'like a sister or a mother to us. When we cried, Susan and I, she would tell us: 'You will get home. You will see your parents again.' She was a very kind person. I will never forget what she did for us."

She was ordered to al-Baghdadi's house to be his secret wife. According to Dalal, "Kayla said he told her: 'I will marry you. If you refuse, I will behead you like your friends.'"

He showed her beheadings of Western hostages, including her friend James Foley.

The girls all worked as servants living in the home of Sayyaf, a senior IS leader when al-Baghdadi summoned Mueller, raped her and made her his wife.

She returned to her shared room after the event weeping, according to Dalal.

Dalal said "After two or three days, he called Kayla again. He gave her a Koran. Every day, he taught her more about Islam. He dressed her in black clothes and he said other men in the house could not look at her."

Dalal and Susan decided to run away, despite Baghdadi's threats, but Mueller refused to join them. She feared that she would be beheaded. She also feared they would be more conspicuous if she joined them, and told the Yazidi girls to leave without her.

The Yazidi girls escaped to Kurdish-held northern Iraq and were rescued.