ISIS hostage Kayla Mueller stood up for her Christian faith while in captivity

US aid worker Kayla Mueller stood up for her Christian faith during her horrific ordeal in ISIS captivity despite claims she had converted to Islam, according to an ABC broadcast.

Fellow-hostages told of how the 25-year-old held on to her hope and faith despite attempts to convert her before she was killed in 2015.

Kayla Mueller, a Christian aid worker taken captive by ISIS, was killed in 2015.Reuters

Dutch photographer Daniel Rye Ottosen, who was taken hostage with Mueller, spoke of how they used to refer to their captors, a group of four Britons who had joined ISIS, including the notorious Jihadi John, as The Beatles.

"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,'" Ottosen said in an interview for the 20/20 The Girl Left Behind programme.

"I would not have had the guts to say that. I don't think so," Rye Ottosen added.

The programme also featured a chilling video of Mueller her parents were sent after her capture in 2013.

"My name is Kayla Mueller. I need your help," she says wearing a black headscarf and a green hijab in the video.

"I've been here too long, and I've been very sick. It's, it's very terrifying here."

Mueller was kidnapped from a Médecins Sans Frontières (MSF) car in Syria and kept as a slave for 18 months. She was repeatedly raped by the ISIS leader Abu Bakr al-Baghdadi, according to her fellow captives.

Negotiations for her release only began 10 months after her capture once MSF, known as Doctors Without Borders in the US, released an ISIS email address two months after they had received it.

But Jason Cone, executive director of MSF-USA, told ABC News that MSF were not hostage negotiators and could not take responsibility for Mueller because she was not an employee.

"There is risk inherent in humanitarian work in conflict, but we rely on people who are willing to take those risks to help us reach people in need around the world," MSF said in a statement.

"It's awful to know that people like Kayla Mueller, who carried a very similar spirit into the world, died during efforts to reach some of those same people."

Mueller's death was confirmed on February 6 last year.