Sri Lanka's president has ordered an investigation into allegations government forces are recruiting children to fight the separatist Tamil Tigers, United Nations envoy Allan Rock said Monday.

Speaking at a news conference in Colombo, Rock, a former federal health and justice minister, said elements within government security forces are recruiting children to fight for the rebel group Karuna, which is battling the Tamil Tigers.

"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]," Rock told a news conference.

"We encountered both direct and indirect evidence of this complicity and participation."

Rock, the UN's special representative for children and armed conflict in Sri Lanka, said Sri Lankan President Mahinda Rajapakse had promised to investigate the allegations.

Rock made the allegations after a 10-day fact-finding trip, said the BBC.

Representatives for the Tamil Tigers and Karuna, a breakaway faction of the Tigers, denied the allegations, say news reports.

The Tigers have been fighting for an independent Tamil state on the island of Sri Lanka since the 1980s. More than 60,000 people have died in their struggle with the Sinhalese government.

Tamil Tigers have used suicide bombers and child soldiers in the conflict, leading Western countries, including the United States and Britain, to label them as terrorists.

Earlier this year, Canada placed the Tamil Tigers on its list of banned terror organizations.