At least two Australians fighting with the extremist Islamic State group have been implicated in sexual slavery in the Middle East.

Islamic State fighters kidnapped hundreds, perhaps thousands of women and girls from the Yazidi minority when they seized control of northern Iraq in the middle of this year.

Since then some have been released after their families paid a ransom, and others have escaped.

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Amena Saeed is a Yazidi activist from northern Iraq. She helped free and resettle some of the victims and said they told her an Australian member of the group was involved at a site across the border in Syria.

"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 amongst the guards, one was from Australia - an Australian man."

While there is no independent proof of the man's identity, Ms Saeed said the girls had no doubt the fighter and captor was Australian and that his nationality was well known.

"The girls had told me this man was infamous because he was one of the Islamic State leaders," she 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."

Ms Saeed's account is backed by Amnesty International, who have said four Yazidi women and girls have identified two Lebanese Australians.

The organisation said they held the Yazidis in their homes, with one already living with his Australian wife and their children.

Amnesty's recent report cites victims who said the fighters separated out married women from those who were single and then concentrated on the youngest and prettiest among them.

Even men who are not fighters with Islamic State have been involved.

Amnesty has been told that local businessmen in Mosul who had good relations with the militia have taken abducted Yazidi girls and registered marriages at the local Sharia court it established.

The thousands of Yazidis who have disappeared in the last four months are amongst Iraq's mass of displaced people with the UN mission's latest report estimating more than two million people have been forced from their homes.