Yazidi women are being bought and sold by Islamic State (IS) militants and forced to marry against their will, Human Rights Watch (HRW) reports.
According to researchers, hundreds of men, women and children from the religious minority, also known as Yezidis, are being held in "makeshift detention facilities" in Iraq and Syria.
Many have been forced to convert to Islam, while others have been sexually abused, raped, and girls in their teens taken as wives for jihadists.
"The Islamic State's litany of horrific crimes against the Yezidis in Iraq only keeps growing. We heard shocking stories of forced religious conversions, forced marriage, and even sexual assault and slavery," special adviser at HRW Fred Abrahams said.
He added that some of the victims are children.
Sixteen Yazidis who managed to escape detention were interviewed by HRW, in addition to two women who remain in captivity but were reached by phone.
They reported that they were abducted by militants on August 3, along with at least several dozen Christians, Shia Shabaks, Turkmen and other religious minorities.
Some interviewees estimated that more than 1,000 people remain in detention.
One 15-year-old girl, Rewshe, was taken to Raqqa, Syria, with a group of around 200 other young women and girls.
Along with her sister, she was sold for $1,000 to a Palestinian who was fighting alongside IS. Her 14-year-old sibling was then sold on, but Rewshe was taken by the man to an apartment where she was forced to fend off a sexual attack.
She managed to escape while the man as sleeping on September 7.
While women face sexual abuse, HRW heard that Yazidi boys are being abducted and radicalised by IS militants.
A 28-year-old man who managed to escape said boys between the ages of eight and 12 were separated from the rest of the group when they were taken from Sinjar.
"The older brothers of those boys became so scared," he recalled.
"They asked, 'Where are you taking them?' They [IS fighters] said, 'Don't worry, we will feed and take care of them. We will take them to a base to teach the Quran, how to fight, and how to be jihadis.'"
HRW has warned that this "systematic abduction and abuse…may amount to crimes against humanity."
"Under international law, crimes against humanity include the crimes of persecution of a religious group, unlawful imprisonment, sexual slavery or any other form of sexual violence of comparable gravity when committed in a systematic or widespread manner as part of the policy of an organized group," a statement reads.
"Islamic State should immediately reunite children with their families, end forced marriages, stop sexual abuse, and release all civilian detainees. International and local actors with influence over the group should press for those actions."