Human Rights Watch has urged Syrian rebel groups to stop enlisting teenagers in their ranks and warned their foreign backers that they could be implicated in "war crimes".
The New York-based rights watchdog accused rebels of using "children as young as 15 to fight in battles" as they try to overthrow President Bashar al-Assad in a bloody conflict that has lasted more than three years.
Some rebel groups recruit teenagers "under the guise of offering education," HRW said in the report published today.
Radical Islamists in Syria "have specifically recruited children through free schooling campaigns that include weapons training, and have given them dangerous tasks, including suicide bombing missions".
The HRW report was based on the experiences of 25 child soldiers - some of whom are still fighting - who were involved in jihadist group the Islamic State of Iraq and Syria (ISIS), also known as ISIL, the Free Syrian Army (FSA), the Islamic Front, Syria's al-Qaeda affiliate the al-Nusra Front as well as with Kurdish forces.
The children interviewed by HRW said they took part in combat, worked as snipers, manned checkpoints, spied, cared for the wounded, or carried munitions or other supplies to the front line.
Many said they had joined up to follow friends or family, while others said they enlisted after taking part in the peaceful demonstrations calling for political change that sparked the conflict in March 2011.
"Syrian armed groups shouldn't prey on vulnerable children - who have seen their relatives killed, schools shelled, and communities destroyed - by enlisting them in their forces," HRW's Priyanka Motaparthy said.
"The horrors of Syria's armed conflict are only made worse by throwing children into the front lines."
The number of child soldiers fighting in the Syrian conflict is unknown, but the Violations Documentation Centre, a organisation close to the opposition, reported that 194 "non-civilian" children have been killed since September 2011.
HRW also urged countries supporting the uprising against Mr Assad to press for an end to child recruitment.
"Governments supporting armed groups in Syria need to press these forces to end child recruitment and use of children in combat," Ms Motaparthy said.
"Anyone providing funding for sending children to war could be complicit in war crimes."
A number said they had joined opposition groups after being personally mistreated by Assad's security forces.
Some of the children interviewed by HRW said they had joined opposition groups after being mistreated by Mr Assad's security forces.
All the children probed were boys, but the Kurdish democratic Union Party also recruits girls to man checkpoints.
The opposition National Coalition has tried to stop the recruitment of teenagers, but FSA commanders say they have continued the practice.
A Kurdish military chief said no one under 18 would be allowed to fight.
HRW said it did not investigate the pro-government militias for its report, citing security and logistical reasons.