The UNs childrens agency, Unicef on Thursday appealed to the Sri Lanka Tamil Tigers to release the child soldiers it says they are holding.
The figure of 1,300 child soldiers was given in a report on child recruitment in 2003.
The report says the Tigers recruited more than 700 child soldiers last year, the youngest being just 10 years old.
Under the terms of Sri Lankas two-year-old ceasefire, the Tigers can still recruit adult fighters.
But they have made several commitments not to take anyone under 18-years of age.
A BBC correspondent says that has been repeatedly flouted – with reports of school children being bundled into vans and driven away by the rebels.
Last year Unicef and the Tamil Tigers agreed an action plan to rehabilitate child soldiers.
However, it is not being expanded as planned because so few children have been released by the rebels to the project.
A transit centre opened by Unicef in rebel territory last October to process released child soldiers now stands empty.
The centre only received 55 children in three months and they have all been sent back to their parents or to live in childrens homes.
Unicef says that until it sees a much greater level of commitment from the Tigers to releasing child soldiers it will not open a second transit centre in the east of the island.
Our correspondent says it is clear the entire project is in jeopardy if the Tigers do not show they are serious about addressing the issue.
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