The UN children's agency has expressed "extreme concern" over reports that 40 children from Iraq's Yazidi minority died after an offensive by jihadists.

Unicef says reports indicate the children died as a "direct consequence of violence, displacement and dehydration" over the past two days.

Thousands of Yazidis fled into the mountains after the Islamic State (IS) overran the town of Sinjar on Sunday.

Yazidis follow an ancient faith that jihadists condemn as devil worship.

"Families who fled the area are in immediate need of urgent assistance, including up to 25,000 children who are now stranded in mountains surrounding Sinjar and are in dire need of humanitarian aid, including drinking water and sanitation services," Unicef said.

BBC Arab affairs editor Sebastian Usher says the Unicef statement goes some way towards confirming some of the most disturbing reports coming out of the heartland of the Yazidi community.

Images posted on the internet showed small clusters of people gathered on the sides of a canyon in the Sinjar mountains.

There have been unconfirmed reports of massacres in Yazidi villages by the jihadists, our correspondent adds.

Jawhar Ali Begg, a spokesman for the Yazidi community, said on Monday that after overrunning Sinjar, IS (formerly known as Isis) had given them an ultimatum to convert to Islam, pay a tax or face death.

BBC