Nearly 300,000 people who were displaced by the ethnic conflict are still held in camps under military guard.
The government insists that it must screen out the remaining rebels before allowing international observers and aid groups free access to the camps.
Nearly 10,000 rebels have surrendered this year, most of them after the defeat of the Tigers in May, said commissioner general of rehabilitation Daya Ratnayake.
"We believe the total number (of rebels in government custody) will be anything between 15,000 to 20,000," he said.
"Depending on their individual needs, we will provide them with basic education and skills training.
He said most of them wanted to be farmers or fishermen, while some wanted careers as teachers or drivers.
Among those who have surrendered were teenagers and rebels who were being trained to carry out suicide bombings, Ratnayake said.