A Belgian ISIS militant with his son in Syria
A Belgian ISIS militant with his son in SyriaVICE/YouTube

Hundreds of children in Syria are reportedly being brainwashed into joining the Islamic State to kill the 'infidels' and the 'Americans'.

In war-torn areas such as Raqqa, which is now under the militant group, the ISIS 'teaching vans' go around recruiting children and training them in the ways of jihad.

"Grooming Children for Jihad", a new documentary released by Vice News, features the footage of an Islamic State teaching van travelling along the Euphrates river in Syria.

"God willing, this generation will fight infidels and apostates, the Americans and their allies, God willing," an unidentified Islamic State militant says in the video..

"The right doctrine has been implanted into these children. All of them love to fight for the sake of building the Islamic State and for the sake of God," the man added.

The documentary crew then goes on to interview several children, as young as five, who pledge their allegiance to the Caliphate and vow to fight and kill Americans.

"I'd like to join the Islamic State in Iraq and Syria and to kill with them," a 14-year-old declares in the interview.

Meanwhile, a 9-year-old tells the crew that he will soon be going to a training camp run by Islamic State to learn how to use a Kalashnikov rifle.

The documentary reveals that the ISIS propaganda, which aims to convert children into agents of jihad.

In one scene, the Islamic State militants could be seen singing along with young boys about 'beautiful virgins' and 'martyrdom', while handing out food to them. In another, the militants are seen conducting comedy shows, before leading the children to pledge their allegiance to the Caliphate.

The depth of ISIS' reach among children was uncovered earlier on Monday, when an image showing the seven-year-old son of Australian jihadist Khaled Sharrouf holding up the severed head of a Syrian soldier made its way online.

US Secretary of State John Kerry said that the image tweeted by Sharrouf is "one of the most disturbing, stomach-turning, grotesque photographs ever displayed."