Nineteen Iraqi Yazidi girls who refused to have sex with their Islamic State captors were placed in iron cages and burned alive in front of a crowd of hundreds in Mosul on Thursday, activists and witnesses say.

“The 19 girls were burned to death, while hundreds of people were watching,” a witness told ARA News. “Nobody could do anything to save them from the brutal punishment.”

ISIS took the girls – and thousands of others -- as sex slaves after overtaking a northern Iraqi region they called home in August 2014.

“They were punished for refusing to have sex with ISIS militants,” local activist Abdullah al-Malla told ARA News.

Human Rights Watch and the United Nations have called on ISIS to release the girls, citing human rights abuses.

“The longer they are held by ISIS, the more horrific life becomes for Yazidi women, bought and sold, brutally raped, their children torn from them,” said Skye Wheeler, a women’s rights emergencies researcher at Human Rights Watch, according to ARA News.

The Kurdistan Regional Government estimates that ISIS in Iraq and Syria is holding 1,800 abducted women and girls, while the U.N. puts the number at 3,500 Yazidis.

Some of the girls have even been put up for sale on social media platforms, according to the Middle East Media Research Institute.

Click for more from ARA News.