Australians fighting with Isis have been identified as the jailers and abusers of abducted Yazidi women forced into sexual slavery.
Islamic State fighters kidnapped thousands of Yazidi women and girls after they swept through northern Iraq earlier this year.
Isis claim it is permissible to enslave Yazidis under their brutal interpretation of Sharia law, as they are "pagans".
Reports indicate many have been raped and forced into marriages with jihadists. Others either escaped or were freed after their families paid ransoms for them.
Yazidi activist Amena Saeed, who helps to re-settle women who were in Isis captivity, told ABC some Australians were complicit in Isis' crimes.
"Most of those held by the terrorists are women and children," she said.
"So far, some 500 have been released. Some of them who were released from this place told us that among the guards, one was from Australia - an Australian man."
She said the man held a senior position in the jihadist group.
"The girls had told me this man was infamous because he was one of the Islamic State leaders," Saeed said. "They stayed two months in Syria after they were transferred from Mosul.
"The Isis leaders used to visit them and forced the women to become Muslims. So they spoke together for a long time and that's also how they know their nationalities."
She said not all were believed to be of Middle Eastern origin.
"I have a case of one escapee who was one of a group of girls bought and raped by two Australian brothers from Isis,' Saeed, a Yazidi former MP, told The Times. 'These were not Australians of Islamic origin - these are white-skinned, blond-haired Australians: converts."
Saeed's claims that some Australians are involved were backed by Amnesty International, who said four Yazidi women and girls identified two Lebanese Australians as captors.
The report by the human rights organisation detailed how Yazidi women and girls were sold in open air markets for as little as £16 to be used as sex slaves.
A former captive, aged 19, described how girls and their mothers were dragged into cattle trucks by their hair before being taken to Mosul, Isis' main stronghold in Iraq, where they were divided according to age and marital status before being forced to convert to Islam and sold off.