A US-based rights group has called on a Tamil Tiger breakaway faction in Sri Lanka to stop using child soldiers.
Rebels have been accused of recruiting children for years
Human Rights Watch said the Karuna group was abducting children for combat with impunity in government-held areas and urged authorities to investigate.
It also said the main Tamil Tiger group was recruiting children. Last month they admitted kidnapping 21 students.
The government denies links with the faction led by Col Karuna, who rejects allegations he uses child soldiers.
Human Rights Watch urged the Sri Lankan authorities to investigate claims that elements in the security forces were complicit in the abductions of children.
The BBC's Roland Buerk in Colombo says Human Rights Watch spoke to 20 families in the east of Sri Lanka and found what it said was evidence of widespread abductions of children to be used as combatants.
"The Karuna group is abducting children in broad daylight in areas firmly under government control," said Brad Adams, the group's Asia director.
"The government is fully aware of the abductions but allows them to happen because it's eager for an ally against the Tamil Tigers."
The organisation says the Karuna faction could have taken hundreds of youngsters, as fighting intensified in the east.
The group broke away from the Tigers in 2004. Its leader rejects suggestions there are any children within his ranks.
"We totally deny this", Col Karuna told the BBC's Tamil service, "because, we have no necessity to abduct children or increase the strength of our forces by inducting them.
"At the same time, we do not work with the government forces."
According to Human Rights Watch, families of missing children said they were taken in areas under government control and sometimes close to checkpoints.
Sri Lanka's security forces have denied they have any links to the Karuna group and government defence spokesman Keheliya Rambukwella said any cases brought to the attention of the authorities would be investigated.
A statement on a government security website criticised UN Special Representative for Children and Armed Conflict Radhika Coomaraswamy, questioning whether she was suitable to oversee investigations into the issue of child soldiers in Sri Lanka.
It suggested that the Karuna faction might in fact have been rescuing underage fighters who had been recruited by the Tamil Tigers.
The rebel Tamil Tigers were also sharply criticised by Human Rights Watch.
They have been accused of recruiting children to use in combat for decades and of continuing the practice.
Last month, 21 girls and boys were taken from a tuition centre in the eastern Sri Lanka by the rebels.
They were later released amid an outcry and the Tigers said junior members of the organisation had made a mistake.
Human Rights Watch has called on both the Karuna faction and the Tamil Tigers to co-operate with Unicef in returning any children in their organisations to their families.
The past year has seen a sharp increase in violence in Sri Lanka, with at least 3,400 people dead, the authorities say.