ISIS is training children
ISIS is training children@abu_dharda/Twitter

A United Nations (UN) watchdog has found that Islamic State (ISIS) militants have used mentally challenged children as suicide bombers.

The report by the the UN Committee on the Rights of the Child, released on Wednesday found that Iraqi boys aged under 18 years 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.

"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," committee expert Renate Winter told Reuters.

Children from not only Yazidi sect or Christian communities, but also Shi'ites and Sunnis have been victims, Winter said.

"It is a huge, huge, huge problem," she told reporters in Geneva. "We are really deeply concerned at torture and murder of those children, especially those belonging to minorities, but not only from minorities."

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."

The report also raised concerns that the Islamic State militants are selling abducted Iraqi children as sex slaves, and killing other youth, either by crucifixion or burying them alive.