The Islamic State jihadist group says that it has given Yazidi women and children captured in northern Iraq to its fighters as spoils of war, boasting it had revived slavery.

The latest issue of its propaganda magazine Dabiq released on Sunday was the first clear admission by the organisation that it was holding and selling Yazidis as slaves.

Tens of thousands of Yazidis, a minority whose population is mostly confined to northern Iraq, have been displaced by the four-month-old jihadist offensive in the region.

Yazidi leaders and rights groups warned in August that the small community faced genocide and that threat was put forward by Washington as one of the main reasons for launching air strikes.

Thousands of Yazidis remained trapped on a mountain near their main hub of Sinjar for days in August, while others were massacred and the fate of hundreds of missing women and children remained unclear.

In an article entitled "The revival of slavery before the hour", Dabiq argues that by enslaving people it claims hold deviant religious beliefs, IS has restored an aspect of Islamic sharia law to its original meaning.

"After capture, the Yazidi women and children were then divided according to the sharia amongst the fighters of the Islamic State who participated in the Sinjar operations," the article said.

"This large-scale enslavement of mushrik (polytheist) families is probably the first since the abandonment of this sharia law," it said.

"The only other known case -- albeit much smaller -- is that of the enslavement of Christian women and children in the Philippines and Nigeria by the mujahedeen there."

Dabiq argued that while the "people of the book" -- or followers of monotheistic religions such as Christians or Jews -- can be given the option of paying the "jizya" tax or convert, this did not apply to Yazidis.

- Bought for $1,000 -

The Yazidi faith is a unique blend of beliefs that draws from several religions and includes the worship of a devil figure they refer to as the Peacock Angel.

In a report also released on Sunday, Human Rights Watch said abducted Yazidi women were subjected to sexual assault and were being bought and sold by IS fighters.

"The systematic abduction and abuse of Yazidi civilians may amount to crimes against humanity," the New York-based watchdog said in a statement.

According to interviews HRW conducted with dozens of displaced Yazidis in the autonomous region of Kurdistan last month and in early October, the jihadist group is holding at least 366 people.

Accounts by some of the Yazidi women who managed to escape and two who are still being held suggest the true number could be at least three times as high.

One 15-year-old girl who escaped on September 7 told HRW that the Palestinian fighter who bought her "told her with pride" that he had paid $1,000 for her.

She said the fighter took her to his flat in the city of Raqa, the group's main hub in Syria, and sexually assaulted her.

Human Rights Watch said that the extent of the sexual abuse inflicted to enslaved Yazidi girls remained unclear but stressed that the stigma surrounding rape in Yazidi culture could explain the low number of first-hand accounts.

"When you ask them, they were never or rarely sexually assaulted. Simply put, they are scared of being killed by their own tribe," Hanaa Edwar, a veteran Iraqi rights activist, told AFP.

"So much harm has been done. There needs to be a huge psychiatric campaign to deal with these victims," she said.