Daesh group has trained more than 400 children in Syria as fighters in 2015 alone, the Syrian Observatory for Human Rights monitor said Tuesday.
Calling them "Cubs of the Caliphate," the jihadist group provides intense military and religious training to children throughout its areas of control in Syria, the Britain-based monitor said.
Sleek videos published by Daesh-affiliated accounts show boys -- some appearing to be as young as eight years old -- loading and firing guns and crawling through sandy brush as part of military training.
The footage also shows children gathered around a table studying religious texts.
Once the boys turn 15, they are given the option to become full-fledged fighters with salaries, said Observatory head Rami Abdel Rahman.
"Daesh officials try to woo children with money, weapons and teaching them how to drive cars," he said.
He said the children aren't forced to fight, but "since there are no (non-Daesh) schools, no work, this is what kids do during the day."
The child soldiers are often used to man checkpoints or to gather intelligence from areas outside Daesh control, as children can typically pass unnoticed through these neighbourhoods, Abdel Rahman said.
But some children are recruited for more violent purposes.
A video released by Daesh this month depicted a boy, who appeared to be no more than 12, executing a hostage by shooting him several times with a pistol.
Abdel Rahman said Daesh had already used at least 10 children as suicide bombers in Syria and had put together all-child battle units.
"This is exploitation and clear brainwashing," he told AFP.
The Daesh group is not alone in using child soldiers in Syria, but it seems to be the group doing so most openly.
"What is striking is they're not hiding it, they're flaunting it," said Nadim Houry, deputy Middle East and North Africa director at Human Rights Watch.
"Their use of child soldiers is part of a broader indoctrination effort... They keep talking about how they'll be the new generation."
Syria's conflict began in March 2011 with peaceful demonstrations, but descended into a civil war that has killed more than 215,000 people.
According to the UN, more than 2.1 million children in Syria are no longer able to attend school due to the violence.