Islamic State militants have burned to death 19 young Yazidi women in Mosul for refusing to have sex with jihadists, ARA News reports.

Eyewitnesses reportedly told the site that the women were forced into iron cages and set alight on June 2 as a crowd of hundreds watched on.

The incident was said to be a punishment for the women, who had been taken as sex slaves by ISIS.

"The 19 girls were burned to death, while hundreds of people were watching. Nobody could do anything to save them from the brutal punishment," an eyewitness told ARA News from Mosul, an ISIS stronghold that was captured two years ago.

Thousands of women and girls were abducted by ISIS militants when the jihadist group invaded the Sinjar region in northern Iraq in August 2014. Hundreds of civilians were slaughtered and more than 400,000 forced to flee. Around 5,000 were taken captive, 3,000 of whom are believed to remain hostage.

Women and girls who have managed to escape have told stories of brutal abuse. They have been bartered and sold for as little as a packet of cigarettes, and kept in dungeons as sex slaves.

In 2014, ISIS issued guidelines specifying how its followers were to treat their female slaves.

A Q&A leaflet detailed exactly how they may buy, sell and abuse "unbelieving" women – thought to mean all those who are not Sunni Muslims.

It clarified that "It is permissible to have intercourse with the female slave who hasn't reached puberty if she is fit for intercourse," and any female slave who attempts to run away is to be "reprimanded in a way that deters others like her from escaping".

Yazidism is an offshoot of Zoroastrianism, which blends ancient religious traditions with both Christianity and Islam. ISIS believes them to be "devil-worshippers".