Children are being abducted, with tacit government approval, from camps housing those displaced by Sri Lanka's ethnic conflict, human rights groups charged on Thursday.

The Coalition to Stop the Use of Child Soldiers said it had received verified reports of child abductions from camps in the main resettlement area of Vavuniya, often by paramilitary Tamil groups.

Children as young as 12 have been among those taken, the coalition said, suggesting that the paramilitaries -- who allied with the military in the fight against the Liberation Tigers of Tamil Eelam (LTTE) -- are being used to identify and weed out former Tiger child soldiers.

The paramilitary groups have been allowed "unhindered" access to the camps which are tightly guarded by government troops, it said.

The Coalition to Stop the Use of Child Soldiers is an umbrella group of global organisations which includes Amnesty International and Human Rights Watch.

"The motive is slightly unclear," said Charu Lata Hogg, a spokeswoman for the groups.

"Some are being taken away for ransom, they've been kidnapped for ransom, and there've been certain negotiated releases where mothers had some jewellery and they could negotiate a release right within the camp," Hogg told the BBC.

"In other cases the children have been taken away for questioning for their alleged links to the LTTE, so they are suspected of being former child soldiers with the LTTE," she said.

The military declared its final victory over the LTTE on Monday, ending a decades-old conflict that has claimed up to 100,000 lives.