ISIS militants are selling abducted Iraqi children at markets as sex slaves, using them as suicide bombers and killing others by crucifixion or burying them alive, a watchdog has said.

The children, who are often from the Yazidi sect or Christian communities, but also Shi'ites and Sunnis, are being tortured and murdered, the U.N. Committee on the Rights of the Child said.

A report released on Wednesday revealed a host of horrifying outcomes for the children kidnapped by the Islamic militants, including boys under-18 being used as bomb makers, informants or human shields to protect facilities against U.S.-led airstrikes.

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Children, who are often from the Yazidi sect or Christian communities, but also Shi'ites and Sunnis, are being sold into sex slavery, turned into suicide bombers or murdered after being kidnapped by ISIS. Pictured: Yazidi refugee children

The U.N. Committee on the Rights of the Child - which produced the report into the children's fates - is calling on the Iraqi government to help rescue those captured by the Islamic militants. Pictured: Yazidi children after they fled from ISIS

Girls are being sold as sex slaves, while children - often with learning difficulties - are being used as suicide bombers.

Others are simply being murdered, the report says.

'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 said.

'The scope of the problem is huge.'

The U.N. 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 [ISIS]'.

This included 'several cases of mass executions of boys, as well as reports of beheadings, crucifixions of children and burying children alive'.

ISIS - which controls land in Syria and Iraq - has committed 'systematic sexual violence', including 'the abduction and sexual enslavement of children', it said.

Winter revealed: '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 also said a number of the children were being used as suicide bombers by ISIS fighters.

Refugee Shyar Ahmad, 14, is one of the children who has been badly wounded due to the war with ISIS militants - another problem highlighted in the report

There is also evidence ISIS is turning its own children into mini-jihadists: this video purported to show a young boy executing two so-called Russian spies

'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,' she said.

'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.'

The report also notes a large number of children have been killed or badly wounded during air strikes or shelling by Iraqi security forces, while others had died of 'dehydration, starvation and heat'.

The 18 independent experts who worked on the report have now called on Iraqi authorities to take all necessary measures to 'rescue children' under the control of Islamic State and to prosecute perpetrators of crimes.