The Islamic State jihadist group says it has given Yazidi women and children captured in northern Iraq to its fighters as spoils of war, boasting it has 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 were trapped on a mountain near their main hub of Sinjar for days in August, while others were massacred.

The fate of hundreds of missing women and children remains unclear.

Fifteen-year-old Rewshe, one of several Yazidi girls who escaped Islamic State captivity and spoke to Human Rights Watch, said Islamic State fighters transported her with about 200 Yazidi women and girls on a convoy of four buses to Raqqa in Syria.

An Islamic State commander sold her and her 14-year-old sister to a fighter, who told her with pride that he had paid $1,000 for her, she said.

The fighter sold her sister to another fighter, Rewshe said. She escaped through an unlocked door while the man who bought her slept.

"The statements of current and former female detainees raise serious concerns about rape and sexual slavery by Islamic State fighters, though the extent of these abuses remains unclear," Human Rights Watch said.

In an article entitled The Revival of Slavery Before the Hour, Dabiq said that by enslaving Yazidis (who it claimed hold deviant religious beliefs), IS had restored an aspect of 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.

"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 mujahideen 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.

The Yazidi faith is a unique blend of beliefs that draws from several religions and includes the worship of a figure called the Peacock Angel.

AFP/Reuters