Christian Aid has "utterly condemned" the rape, sex slavery and murders of Yazidi women in Iraq.
Evidence of the atrocities is emerging in reports of women being held in an Islamic State prison where they are being sold for between $25 and $150 as brides.
The charity is among the humanitarian groups and aid agencies who are increasingly concerned about the women, taken captivity while attempting to escape Mount Sinjar.
They have been imprisoned in Badush Prison, where hundreds of Shia prisoners were executed when Islamic State first took Mosul, in northern Iraq.
The Daily Beast reported that women and girls as young as 14 are being offered the chance to convert. If they accept, they are sold as wives. If not, they are raped, tortured and eventually murdered.
Women from other groups, including Christians and Turkomans, are also thought to be suffering the same fate, according to sources in Iraq.
Accounts of the prison have come from women who managed to hide their mobile phones, calling relatives to describe their plight. Some imprisoned women have been forced by militants to call their families.
The mother of one woman still held captive told The Daily Beast about the call she received from her daughter. She was forced to listen as her daughter detailed being raped by dozens of men over the course of a few hours. Babies are being born in the prison, and being immediately taken away from the mothers who have no idea of their fate.
Pakhshan Zangana, head of the High Council of Women's Affairs for The Kurdish Regional Government, who is attempting to help by raising funds to try and buy the captured Yazidi women back into safety, said: "We have women and families calling in every day, the situation is getting desperate."
She added: "It's sick. [Islamic State] went so far as to force the local beauticians to come in and dress them up, putting makeup on them. Then telling them to instruct the women to be submissive to their new husbands." She added: "This is not just a Kurdish or Iraqi problem, this is an international crisis."
Ann Ward, Christian Aid's consultant in Iraq, who has met many Yazidi communities affected by the recent violence, said: "I have heard stories from Yazidi families whose female family members have been abducted by IS."
Christian Aid said: "We are appealing for the protection of all people, including persecuted religious minorities such as the Yazidis and Christians – hundreds of thousands of whom have fled in fear of their lives.
"Our long-term partners in Iraq, who have already been responding to the ongoing displacement of communities, are providing vital humanitarian relief to communities displaced in recent weeks. This has been done through the kind support to the Christian Aid Iraq appeal.
"Christian Aid utterly condemns the deliberate targeting of civilians and committing of atrocities against civilians."