Many children in Iraq remain at the mercy of ruthless armed groups that use them as fighters, suicide bombers and human shields and subject them to systematic abuse, a UN watchdog said Wednesday.

In a report on the plight of children in the strife-torn country, a UN committee voiced deep concern over "the large number of children recruited by non-state armed groups," and especially the Islamic State jihadists.

"It is a huge, huge, huge problem," Renate Winter, a member of the United Nations Committee on the Rights of the Child (CRC), told reporters in Geneva.

IS spearheaded a sweeping offensive that has overrun much of Iraq's Sunni Arab heartland since June, carrying out a campaign of brutal killings, kidnappings and torture.

Children have not been spared.

CRC, which is composed of 18 independent experts who monitor the implementation of international children's rights treaties, denounced numerous cases of IS militants torturing and murdering children, especially those from minorities.

The group has been targeting and attacking schools, executing teachers and subjecting children to systematic sexual abuse, including sexual slavery, the committee said.

"Children (are) being used as suicide bombers, including children with disabilities or who were sold to armed groups by their families," said the report, which also detailed how children were used as "human shields" to protect IS facilities from airstrikes, to work at checkpoints or build bombs for the jihadists.

The committee has urged Baghdad to explicitly outlaw the recruitment of anyone under the age of 18 into armed conflict.

While the Iraqi government is responsible for protecting its citizens, Winter acknowledged it was probably powerless at present to hold the jihadists accountable.

She said the government should strive to do as much as possible to protect children in areas it controls and do everything it can to rescue youths from IS-controlled territory.

However, the committee took Baghdad to task for a number of abuses that cannot be blamed on the jihadists, including reports of underaged boys used to guard government checkpoints and children held in harsh conditions on terrorism-related charges.

It also denounced frequent honour killings as well as forced early and temporary marriages of girls as young as 11.

The committee took particular issue with a law that allows rapists to go free if they marry their victim, rejecting Baghdad's argument that the law was "the only way of protecting the victim from reprisals of her family."