Elements in the Sri Lankan military are helping a breakaway rebel faction to abduct children as soldiers to fight Tamil Tiger rebels, the UN has said.
Children have long been caught up in Sri Lanka's conflict
A senior UN official said there was "credible evidence" that troops had rounded up children to fight with the renegade rebel group led by Col Karuna.
His faction split from the Tamil Tigers, long accused of using children.
Sri Lankan security forces say they are "perturbed" by the "completely misleading" allegations.
A Karuna spokesman also denied the allegations, saying his group merely offered protection to children fleeing fighting with the rival Tamil Tigers.
But Allan Rock, a special adviser to the UN representative for children and armed conflict, said government forces had forcibly rounded up young Tamil children to fight with Col Karuna's group.
"We encountered both direct and indirect evidence of... complicity and participation," he said of the government security services.
The BBC's Dumeetha Luthra, in Colombo, says the allegation, the first of its kind made by the UN against the Sri Lankan military, follows a 10-day fact-finding mission.
The army has long denied allegations that it actively supports the efforts of the rebel faction led by Col Karuna, following his split from the Tamil Tigers in 2004.
Mr Rock spoke of 13 and 14-year-old children being kidnapped from villages, and no arrests or investigation being carried out by the security forces.
He said there was both eyewitness and anecdotal evidence to back up his claims.
In a statement the Sri Lankan Armed forces said Mr Rock's claims that government troops were actively involved in the recruitment of child soldiers were "regrettable".
"Security forces... vehemently deny having any involvement whatsoever with the LTTE breakaway group for abductions in Batticaloa."
Sri Lankan President Mahinda Rajapakse has promised a full investigation into the allegations.
Mr Rock said the fact that Sri Lankan troops were complicit in the recruitment of child soldiers meant that Tamil Tiger rebels would continue to do so, as it corroded the rule of law.
At least 2,000 people have been killed in violence this year in Sri Lanka, the military and ceasefire monitors say.
The Tamil Tigers are fighting for an independent homeland in the north and east of the country, and claim that ethnic Tamils have suffered decades of discrimination at the hands of Sri Lanka's Sinhalese majority.