Iraqi boys aged under 18 are increasingly being used by the militant group as suicide bombers, bomb makers, informants or human shields to protect facilities against US-led air strikes, the UN Committee on the Rights of the Child said.

"We are really deeply concerned at torture and murder of those children, especially those belonging to minorities, but not only from minorities," committee expert Renate Winter told a news briefing. He added :"The scope of the problem is huge."

Children from the Yazidi sect or Christian communities, but also Shi'ites and Sunnis, have been victims, she said.

"We have had reports of children, especially children who are mentally challenged, who have been used as suicide bombers, most probably without them even understanding," Winter told Reuters.

"There was a video placed (online) that showed children at a very young age, approximately eight years of age and younger, to be trained already to become child soldiers."

ISIS has killed thousands and forced hundreds of thousands from their homes, in what the United Nations has called a reign of terror.

On Tuesday, the group, which is also known as ISIL, released a video showing a captured Jordanian pilot being burned alive.

The UN body, which reviewed Iraq's record for the first time since 1998, denounced "the systematic killing of children belonging to religious and ethnic minorities by the so-called ISIL, including several cases of mass executions of boys, as well as reports of beheadings, crucifixions of children and burying children alive".

A large number of children have been killed or badly wounded during US-led air strikes, while others had died of "dehydration, starvation and heat", it said.

ISIL has committed "systematic sexual violence", including "the abduction and sexual enslavement of children", it said.

"Children of minorities have been captured in many places... sold in the market place with tags, price tags on them, they have been sold as slaves," Winter said, giving no details.

The 18 independent experts who worked on the report called on Iraqi authorities to take all necessary measures to "rescue children" under the control of ISIS and to prosecute perpetrators of crimes.