The Islamic State is training young boys for battle, according to evidence from residents, activists and human rights groups.

Several activists told the Associated Press that they saw children fighting beside the extremist militants in the northern Syrian town of Kobani.

A Kobani-based activist, Mustafa Bali said he saw the bodies of four young boys where two of them were under the age of 14. The activist also said that at least one 18-year-old carried out a suicide attack.

Boys attend training camps and take religious courses before they head out to fight in the Islamic State's de facto capital in Syria, Raqqa. Some of the boys also work as spies for the extremist group or cooks and guards at the IS headquarters.

There is no exact figure on how many minors are recruited mainly because the IS territory is tightly controlled.

The group "prioritizes children as a vehicle for ensuring long-term loyalty, adherence to their ideology and a cadre of devoted fighters that will see violence as a way of life," the United Nations panel investigating war crimes in the Syrian conflict said in a recent report.

AP reported that the panel of experts, known as the Independent International Commission of Inquiry on Syria, conducted more than 300 interviews with people who fled or are living in IS-controlled areas, and examined video and photographic evidence.

Although recruiting children for battle is not a new subject, IS uses children in a systematic and organized way that has rarely been seen before.

"What is new is that ISIS seems to be quite transparent and vocal about their intention and their practice of recruiting children," UNICEF regional child protection adviser for the Middle East and North Africa Laurent Chapuis told the AP.

"Children as young as 10, 12 years old are being used in a variety of roles, as combatants as messengers, spies, guards, manning checkpoints but also for domestic purposes like cooking, cleaning, sometimes providing medical care to the wounded."

While some children are targeted by the extremists, others join voluntarily.