British ISIS members say they were jailers of Christian aid worker Kayla Mueller

Kayla Mueller, a Christian aid worker, died while being held by ISISReuters

More light has been shed on the horrific ordeal of a young Christian woman who was raped and tortured by ISIS militants before being killed.

In a video obtained by US news channel NCB, British ISIS members Alexanda Kotey and El Shafee Elsheikh shared details about the imprisonment of 26-year-old American, Kayla Mueller, who was an aid worker when she was taken hostage.  She died in 2015. 

Kotey, 36, and Elsheikh, 31, are being held by the US military at a base in Iraq. 

They were part of a group of militants nicknamed the Beatles because of their English accents.  The group took prisoners for ransom money but many were killed when their demands were not met.

Describing Mueller's captivity, Kotey said: "She was in a room by herself that no one would go in." 

Elsheikh told of how he "took an email from her myself" - understood to mean that he extracted an email address from Mueller to send a ransom demand to. 

He also described her as being kept in a room by herself.

"She was in a large room, it was dark, and she was alone, and . . . she was very scared," he said.

Mueller was kidnapped in Syria after visiting a Doctors Without Borders hospital and held for 18 months, during which time she was repeatedly raped by ISIS leader Abu Bakr al-Baghdadi.  

In 2016, former fellow hostages said that Mueller had stood up for her Christian faith during her captivity. 

"One of the Beatles started to say, 'Oh, this is Kayla, and she has been held all by herself. And she is much stronger than you guys. And she's much smarter. She converted to Islam.' And then she was like, 'No, I didn't,'" said Dutch photographer Ray Ottosen, who was held alongside her. 

Speaking in an interview for the 20/20 The Girl Left Behind programme, he added: "I would not have had the guts to say that. I don't think so."

Her father Carl told the Daily Mail it's been hard to hear the details of her ordeal. 

"It's not easy to go through this, it's been five years since Kayla has been gone, a year and a half before that she was captured, it's hard, very hard," he said.