In a report released on Wednesday, the U.N. Committee on the Rights of the Child said it had received reports of "several cases of mass executions of boys, as well as reports of beheadings, crucifixions of children and burying children alive."

"Iraq's military has been struggling in its fight against Islamic State, and the government has pleaded repeatedly for more weapons and training.

"Still, the U.N. agency urged that more be done, saying Iraq needs to 'take all necessary actions to ensure the safety and protection of children and their families.'"

Reuters reports that investigators found ethnic minority children were most at risk in the areas of Iraq controlled by the ISIS.

Committee expert Renate Winter told reporters the "scope of the problem is huge."

He said that mentally challenged children, for example, have been used as suicide bombers.

"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," Winter said, according to Reuters.

Winter added that ISIS was also committing "systematic sexual violence."