Islamic State militants continue to defy international humanitarian laws by selling abducted Syrian and Iraqi children as sex slaves, torturing them, and crucifying and burying them alive, a United Nations watchdog reported on Wednesday.

The abducted children are mostly from Christian communities and some are Shi'ites and Sunnis.

"We are really deeply concerned at the torture and murder of those children, especially those belonging to minorities, but not only from minorities. The scope of the problem is huge," UN Committee expert Renate Winter told the press.

According to Reuters, the UN Committee on the Rights of the Child said that the militants have been using Iraqi boys aged under 18 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," Winter 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."

Human Rights Watch has previously reported the torture and beating of abducted Kurdish children. There were reportedly 250 children taken from Kobani. Testimonies of some who managed to escape showed the conditions they were kept under.

"I was once put inside the tire and beaten. They sometimes found excuses to beat us for no reason...They made us learn verses of the Quran and beat those who didn't manage to learn them. When some boys tried to escape, the treatment got worse and we were all punished and given less food," a 16-year-old boy reports.

ISIS has been spreading terror across Syria and Iraq persecuting Christians to convert to Islam and killing those who refused. On one occasion four children, all under 15, were beheaded after refusing to denounce their faith in Jesus.

After further investigation by the UN body, they called on Iraqi authorities to take all drastic measures to rescue children from the hands of the ISIS militant group.

"There is a duty of a state to protect all its children. The point is just how are they going to do that in such a situation?" Winter said.