The voice pledging jihad in the video is brash, prideful and chillingly juvenile.

"We promise you car bombs and explosives," the boy shouts, his small arms and hands gesturing wildly for emphasis.

"We'll destroy the enemies of the religions, all of them — all who fought the Islamic State," he continues.

"And the Caliphate?" prompts an adult male voice, coaching the youth off-camera.

"And the Caliphate. The Caliphate will remain until the end of the world," the child answers, reiterating his support for Abu Bakr al-Baghdadi, leader of Islamic State of Iraq and Syria (ISIS).

The boy, who appears to be not much older than 12 in the Vice News documentary, embodies what human-rights and counter-terrorism researchers describe as a sophisticated and dangerous new phase for the Sunni extremists — the cultivation of child jihadis as part of a long-term strategy to building a caliphate, or Islamic state, in Iraq and Syria.

"They want the children because the children can be moulded," said John Rossomando, a senior analyst with the Washington-based Investigative Project on Terrorism.

An Australian man posted photos of himself and young armed boys believed to be his sons in front of the ISIS flag in Syria. (The Australian/Twitter)

"They're easy to seduce. They're using children's images to say, 'Even our next generation is coming for you,'" Rossomando said.

ISIS isn't shying away from using images of underaged fighters to market jihad abroad.

Rather, children are "predominantly displayed" in their propaganda, said Bill Roggio, a senior fellow at the Washington-based Foundation for Defense of Democracy.

One video of an ISIS convoy moving through Mosul in northern Iraq shows a baby-faced soldier toting an assault rifle in the carriage of a truck.

The leader of the camp said [ISIS] liked the younger ones better. He told me, ‘Tomorrow they’ll be a stronger leader or a stronger fighter.' - Former ISIS child soldier, as told to Human Rights Watch

Tweets have also purported to show Iraqi children in ISIS-held territories flashing the terrorist group's hand salute, or patrolling the streets with guns.

This week, an Australian grandfather pleaded with local politicians to help retrieve his grandson, after the child appeared in a video from Syria with his father hoisting up the decapitated head of a soldier.

Roggio, who tracks jihadist propaganda, said photos have been distributed showing militants "handing out ice cream or having carnivals" in ISIS-controlled Raqqa in northern Syria.

"It's like saying, 'Look, we're popular amongst a broad range of society. Even the children love us, and we're here to raise your children to be good Muslims,'" he said. "They're trying to be family-friendly jihad."

Human Rights Watch, which interviewed 25 former child fighters who took up arms in Syria, reported in June that armed rebels groups in the battle-scarred country have drafted boys as young as 15 for training.

As of June, the Violations Documenting Center, a Syrian monitoring group, said it has logged 194 cases of "non-civilian male children" who died while fighting since the civil war erupted in September 2011.

'Signed up for suicide missions'

One section of the HRW report that focused on ISIS stated the group "reached out to young people, including children, in systematic ways, entering schools and providing education in mosques that includes weapons and military training."

Some of the children who eventually left the terrorist outfit did so because they were wounded in battle or captured by government forces, said Jo Becker, HRW's children's rights advocacy director.

This image of a child, purportedly a nine-year-old ISIS recruit from Syria, was shared on social media.

Becker, who has worked on child soldier issues for the past 17 years, said some of the male children were enticed to join the extremist cause after attending speeches at public mosques and other forums. Once recruited, they were used for spying, trained for combat, and even received military uniforms, bulletproof vests and salaries.

"At least one [former child soldier] reported that children in his unit had signed up for suicide missions," she said.

Raed, now 17, said he joined the training camp less than two years ago because he liked what the militants were wearing, and he admired their "one herd" mentality.

"The leader of the camp said [ISIS] liked the younger ones better," Raed said in the HRW report. "He told me, ‘Tomorrow they’ll be a stronger leader or a stronger fighter.’"

The indoctrination of children, who Becker said are easy prey for jihadists because they haven't developed mature judgment, could be of particular concern because of what that says about the scale of ISIS's ambitions.

Boys with 'desire to go to battle'

Although youths may not yet be the most capable fighters, a potentially more dangerous prospect is a boy who has had years to mature into a deadly adult combatant on the battlefield.

Some analysts have argued that, unlike child soldiers in some parts of Africa who might be sent into battle against their will, stolen from their parents or drugged to become fearless killers, some young ISIS fighters appear to have enlisted on their own volition.

Article 38 of the UN Convention on the Rights of the Child: Read more here about what Article 38 of the UN Convention on the Rights of the Child says about the use of child soldiers.

HRW said that children who joined armed groups in Syria did so after enduring torture by the regime, after participating in protests, or because schooling was no longer an option.

But, according to the report, others "simply … had a desire to go to battle."

"The images of child soldiers I’m most familiar with are out of Africa, where there’s the practice of taking young children and sort of breaking them down psychologically and remoulding them as trained killers said Michael Dartnell, a terrorism expert who teaches political science at Georgian College in Ontario.

"I’m not certain if that’s actually what is going on in Syria and Iraq right now."

Even so, Dartnell added he wasn’t sure about the capacity for such young minds to make independent ideological choices so early in life.

While Roggio said his monitoring of jihadi social-media feeds brought up images of young ISIS supporters brandishing weapons, marching in extremist parades and hitting posters of perceived infidels with their shoes, "I don’t see them being deployed on the battlefields or into towns" as one might expect a child soldier to do.

Instead, he believes youths are considered by ISIS as being more of a long-term "asset."

"The children aren't disposable to them," he said, adding that their exploitation is a means of survival for the movement.

Becker, with HRW, said that child soldiers have been shown in the past to be used in some of the most dangerous tasks, such as suicide missions.

"It’s deeply concerning," she said. "All children in Syria are suffering incredibly from the violence there, but to put children directly into battle is just beyond the pale."