A window has been opened into the secretive world of the Islamic State (ISIS), showing how the extremists are training children to fill their depleted ranks as they come under relentless attacks from Iraqi and other coalition forces.

Sky News was given a rare access into the ISIS training camps courtesy of a 15-year-old would-be suicide bomber named Mahmoud Ahmed who revealed what he knew about the terrorist organisation.

Ahmed was ordered by his ISIS handlers to carry out an attack on a sports stadium in Kirkuk, Iraq in August this year. However, he hesitated in carrying out the order given him, leading to his arrest. The boy is now held at an undisclosed juvenile detention centre in Iraq on terrorism charges.

Speaking to a Sky News reporter in the presence of Kurdish intelligence agents, Ahmed said he was one of dozens of other children, called Cubs of the Caliphate, who were placed in a special ISIS camp.

He described how he was radicalised and given weapons training. "They taught us how to use a Kalashnikov and a PKC machine gun," he said. "There were four older men who would teach us about heaven and stuff like that. Twenty-four hours a day they'd teach us about this stuff. There were 60 of us born from 2002 onwards."

Ahmed said their handlers tried to scare them and showed them videos of beheadings.

Asked whether he knew what he was doing was wrong, Ahmed recalled the incident in August when he was ordered to enter a football stadium and blow himself up.

"When I reached the target I knew it was wrong. When I saw the young kids I knew it was wrong immediately.

"I returned back to him (my handler) but he said go straight back. I told him no and he said this is an order from Abu Islam (an ISIS commander).

"I returned back around the stadium and they caught me," Ahmed said.

Meanwhile, Kurdish intelligence officials have revealed that Ahmed is just one of thousands of children across Iraq and Syria who are being trained by ISIS to fight and carry out suicide attacks. Some of the children are as young as nine years old, the officials added.

A Kurdish intelligence officer said ISIS is using child fighters and suicide bombers because they are harder to detect by security forces and they are also easily duped.

"They are very vulnerable. It is much easier to turn a child into a suicide bomber than a grown-up," the officer said.