The Islamic State group is using children, including those who are mentally challenged, as suicide bombers in Iraq while also selling those they have abducted as sex slaves and killing others by crucifixion and burying them alive, a United Nations watchdog said, in a report released Wednesday.

According to the U.N. Committee on the Rights of the Child, ISIS militants are using Iraqi boys below the age of 18 as informants or human shields to protect their facilities against U.S.-led air strikes. The committee also urged the Iraqi government to act to ensure the safety and protection of the children, the Associated Press reported.

“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,” Renate Winter, a committee expert, 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.”

The U.N. body also condemned ISIS for killing children belonging to religious and ethnic minorities. According to the committee, there have been several cases of mass executions of children by ISIS, “as well as reports of beheadings, crucifixions of children and burying children alive.”

Many children were also killed or severely wounded during airstrikes while others had died of dehydration, starvation and heat, the report 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,” Reuters quoted Winter as saying.

The extremist group recently released a propaganda book, in which it urged Muslim mothers to start training their children in the ways of jihad. The book, titled, “Sister's Role in Jihad,” asked mothers to get their children books, CDs and videos on conflict, and recommended that they visit websites on the subject along with their children, and utilize "other internet resources.”

Recent media reports have noted that parents in the Syrian city of Raqqa were giving up their children to ISIS. Reports also said that school kids in Raqqa could become suicide bombers in as little as 45 days after being taken by ISIS.

“We are really deeply concerned at torture and murder of those children, especially those belonging to minorities, but not only from minorities,” Winter said. “The scope of the problem is huge.”