A Yazidi woman who was held as a sex slave by ISIS has asked the world to unite in the fight against the terrorist group during a speech in London.

Nadia Murad, 21, was orphaned by ISIS, who killed her mother and six brothers in Sinjar, Northern Iraq, and taken by the group and kept as a sex slave.

Murad was among more than 5,000 Yazidi women taken captive when ISIS took Sinjar. More than 3,400 remain in captivity, she said.

"A year and a half has passed and the genocide against the Yazidis is continuous. We die every day because we see the world silent in the face of our plight," she said, according to the Mirror.

Describing the massacre of her family, Murad said: "My mother saw them killing my brothers and then they took my mother and killed her.

"I was already orphaned as I didn't have a father, all I had in the war was my mother," she said.

"But when they took me to Mosul and raped me, I forgot my mother and brothers. Because what they were doing to the women was more difficult than death.

"Imagine until now, for more than a year and a half, girls as young as nine are being rented and sold [for sex]."

Murad has been campaigning for two months since her release seeking to raise awareness of the plight of the Yazidi people."When I speak I didn't speak just on my behalf, but on behalf of all the women and children affected in the war zone," she said.

Yazidism is an offshoot of Zoroastrianism, which blends ancient religious traditions with both Christianity and Islam. According to ISIS doctrine, Yazidis are "devil-worshippers", and many have been systematically persecuted by militants.

"About 5,800 Yazidi women and children were captured by the so-called Islamic State. They have killed many people in Iraq and Syria and displaced millions," Murad said.

"For us, the Yazidis, they killed the men and took the women and children. They were committing all kinds; murder, rape and displacing people by force in the name of Islam. Many people may think my story is difficult, but many more had more difficult than mine. They killed six of my brothers, but there are families that have lost 10 brothers," she said.

"At this moment, there are still 3,400 women in the hands of ISIS. All we are asking is that our women are freed."

During her speech as the Trade Union Congress House in London, she branded ISIS "criminals" and compelled the global community to "come on board" the fight against them.