KATHMANDU, May 8 (Reuters) - Nepal's former Maoist rebels should free thousands of child soldiers from their ranks now that they have joined the political mainstream, Human Rights Watch said on Tuesday.

The once-feared guerrillas signed a peace deal in November under which they confined 31,000 fighters to camps and joined an interim parliament as well as a coalition government.

Between 6,000 and 9,000 of the Maoist fighters, who are housed in 28 camps monitored by the United Nations, are believed to be children under the age of 18, the New York-based rights group said.

"There is no excuse for letting children languish in cantonment sites month after month," Jo Becker, the group's advocate for children, said in a statement.

"Under the terms of Nepal's peace agreement, these children should be released immediately so they can enter rehabilitation programmes, get back into school, and rejoin their families."

Last week, the U.N. was supposed to begin the verification of the Maoists in camps to see if their ranks included children or new recruits who had joined after the peace process began last year, as has been alleged.

But the world body said the Maoists had prevented them from starting the verification, in a violation of the terms of an agreement on the management of arms and armies.

The former rebels say the government should first give them more money to improve conditions at the camps, complaining that many lack water and electricity.

In February, a Human Rights Watch report chronicled how children as young as 14 served on the front lines, received weapons training, and carried out crucial military and logistical support duties for the Maoists.

On trips to the countryside, Reuters has also seen young children working in the Maoist militia and found evidence that the rebels had tried to boost their numbers by recruiting new cadres after a ceasefire was agreed last May.

More than 13,000 people died in a decade-long civil war that also badly undermined impoverished Nepal's aid- and tourism-dependent economy.

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