A teenage girl who was abducted by ISIS and used as a sex slave has spoken out about her experience, and having to leave her child born as a result of rape behind when she escaped.

Nihad Barakat Shamo Alawsi was 15 when she was abducted by Islamic State militants in the Yazidi town of Sinjar in northwestern Iraq in August 2014.

She and 27 members of her family were among around 5,000 Yazidis to be kidnapped that summer. She was taken to Syria and then to ISIS' stronghold, Mosul.

"They raped us, they killed our men, they took our babies away from us," Alawsi, now 17, said at the event organised by the UK-based AMAR Foundation, a charity that provides education and healthcare in the Middle East.

"The worst thing was the torture in Mosul. We were beaten and raped continuously for two weeks," she said "Girls were taken from their families and raped constantly and then they were handed out to 'emirs'".

Yazidism is an offshoot of Zoroastrianism, which blends ancient religious traditions with both Christianity and Islam. ISIS believes them to be "devil-worshippers". Of the 5,000 Yazidi men and women captured in 2014, around 2,000 have since been smuggled out or managed to escape, and harrowing accounts of their treatment at the hands of militants have emerged.

Alawsi said a man who took her as a slave died a few weeks later, and she was sold to another man who already had a wife and another Yazidi sex slave. He beat and raped her and a month later she became pregnant.

"I thought the child I was carrying was a member of Daesh and would become a Daesh criminal when he grew up," Alawsi said, using a pejorative Arabic name for Islamic State.

Alawsi gave birth to a boy, but three months later she managed to escape after the baby's father decided to marry her to his cousin.

"I managed to make a phone call to my family with someone's help, and I managed to escape, but I had to leave the baby behind," she said.

Most of the Yazidi population, numbering around half a million, are displaced in camps in Iraq's northern Kurdistan.

Alawsi now lives in one of the camps with her mother, father and siblings, and works with AMAR, volunteering to come to London to speak of her people's plight. Two of her brothers and two sisters are still held by Islamic State.

"It's not a life, we are not living a life until the rest of our people are released by Daesh," Alawsi said.

"I beg you to help my people, to save them from Daesh, and to free especially the sex slaves, the young girls and children that have been taken."

Additional reporting by Reuters.