The UN said that 135 children had been abducted by the Karuna faction - a group that broke away from the Tamil Tigers and is now suspected of working with the military - since May in the eastern district of Batticaloa, and that there was "evidence that this trend is accelerating".
Allan Rock, a special adviser to the UN special representative for children and armed conflict in Sri Lanka, said: "One very disturbing element that confronted us ... has to do with the complicity and participation of some elements of the government's security forces in the forcible abductions by Karuna of children [in the east].
"We encountered both direct and indirect evidence of this complicity and participation."
Rock said that Mahinda Rajapakse, the Sri Lankan president, had pledged to investigate the allegations immediately and punish those responsible.
The military released a statement saying it "vehemently denies having any involvement whatsoever with the LTTE [Liberation Tigers of Tamil Eelam] breakaway group for abductions in Batticaloa".
Rock said Karuna and the Tigers
continued to recruit children
The breakaway group is named after their leader, who is known as Karuna.
He took about 6,000 fighters with him when he split from the Tamil Tigers in 2004.
Rock also said the Tigers had continued to recruit child combatants and had failed to honour a pledge to release them.
They have promised to free all their underage fighters by January 1, 2007.
Unicef says there are 1,598 outstanding cases of underage recruitment by the Tigers, 649 of whom are still under the age of 18. The agency also lists 142 cases by the Karuna group.
Rock said he suspected the real number of underage recruits was far higher.
The Tamil Tigers have been fighting the government since 1983, demanding a separate homeland for ethnic minority Tamils in the north and east of Sri Lanka.
The defence ministry says 3,289 people have died in the fighting this year - 860 government security personnel, 549 civilians and 1,880 separatists.
The Tigers do not usually give casualty figures.