The United Nation's children agency, UNICEF on Thursday said the Liberation Tigers of Tamil Eelam (LTTE) recruited 709 underage fighters last year to bring the number of child soldiers in their ranks to 1,301 at the end of December.
"The LTTE must cease all recruitment of children and they must release all child soldiers immediately," UNICEF representative Ted Chaiban said.
The fresh recruitments of child soldiers took place even while the LTTE observed a Norway-brokered truce with the government in Colombo.
The disconcerting UNICEF report came amid a breakdown in peace talks and growing fears of a fresh outbreak of hostilities between government forces and the rebels.
Engaged in a vicious insurgency since the last 20-years, LTTE is seeking a separate homeland for the minority Tamils in the country's north.
It threatened a renewed war on Tuesday after Sri Lankan President Chandrika Kumaratunga entered into an alliance with a "radical party" that opposes autonomy for the ethnic Tamils.
"The LTTE must cease all recruitment of children and they must release all child soldiers immediately"
Thousand have died in Sri Lanka's bloody war. The sizeable presence of underage soldiers is yet another brutal facet of the unresolved conflict.
The rebels are reportedly continuing to recruit child soldiers despite public pledges to desist from it. In October last, the group released 49 underage soldiers and sent them to a UNICEF-run rehabilitation home.
The UNICEF representative said an "Action Plan" to help children in the country's embattled northern and eastern regions could not succeed unless the rebel Tigers honoured their pledges not to recruit children.
UNICEF's three-year $14.2 million Action Plan hopes to provide the first formal mechanism for the release and reintegration of child soldiers.
According to the UNICEF, some 50,000 children of the troubled northern and eastern regions were out of school while 140,000 have been displaced from their homes.
Landmines have killed 20 children and maimed 17 others during 2003 alone.
There was also an estimated shortage of 6,000 teachers in the embattled regions.