* Children as young as 15 sent into battle
* Youths man checkpoint, act as snipers, tend to wounded
* Children found among main opposition groups
The report said the Islamic State in Iraq and the Levant (ISIL), which has made rapid territorial gains across the border in Iraq, had given children weapons training in Syria and told them to carry out suicide bombings.
Citing personal accounts, the rights group also found evidence of children being mobilised by the more moderate Western-backed Free Syrian Army, the al Qaeda-affiliated Nusra Front, the Islamic Front coalition and security forces in Kurdish-controlled areas.
"The horrors of Syria's armed conflict are only made worse by throwing children into the front lines," said Priyanka Motaparthy, the author of the report which drew on the accounts of 25 children. It said 14-year-old youths had been used in support roles for the fighting.
Reuters could not independently confirm the accounts. The British-based Syrian Observatory for Human Rights, an anti-Assad monitoring group, said on Sunday that relatives of kidnapped students in Syria fear that ISIL will use the children to carry out car bombs or suicide attacks.
Syria's conflict started with peaceful demonstrations for political change in 2011 but has descended into a civil war, pitting forces loyal to President Bashar al-Assad against a myriad of opposition groups.
Infighting among opposition combatants has complicated the conflict, which has stirred sectarian tensions across the Middle East and spilled over into neighbouring countries.
HRW said the number of children fighting in Syria was not known. The Violations Documenting Center, a Syrian monitoring group, had documented 194 deaths of "non-civilian" male children in the country since September 2011, the report said.
"ALLAH CHOSE YOU"
The children said they had fought in battles, acted as snipers, manned checkpoints, treated the wounded on battlefields, and brought supplies to front lines, the report said.
A 16-year-old boy who gave his name as Majed said the Nusra Front recruited him and other boys in the southern city of Deraa near the Jordanian border.
The group provided free schooling at a local mosque that included military training and commanders had asked children as well as adults to carry out suicide attacks, he said.
"Sometimes the commanders would say, "Allah chose you," and sometimes the fighters volunteered," Majed said, according to the report.
Many children followed their relatives or friends into the armed groups, while others lived in battle zones without schooling or other options, HRW said. Others had taken part in protests or were angry with the government.
All of the 25 interviewed were boys, but the Kurdish Democratic Union Party (PYD) police force and its armed wing had enlisted girls to guard checkpoints and conduct armed patrols in Kurdish-controlled areas, the report said.
Human Rights Watch said some armed groups had taken steps to end the use of children in the conflict. The Western-backed Syrian National Coalition said it was examining the allegations.
"We take such allegations extremely seriously and are committed to ensuring that anyone responsible for the voluntary or involuntary recruitment of children is held to account," it told Human Rights Watch in a letter.
The Islamic Front, a coalition of several rebel factions, told HRW it had investigated the accounts and found no evidence that its fighters included children.
"We never arm a young man, or give him the opportunity to join the Front, including the Ahrar Al-Sham Islamic movement, except after a thorough check of his documents, to ascertain he's over 18," it told HRW.