By Frances Harrison BBC correspondent in Colombo

The London-based human rights organisation Amnesty International has accused Sri Lanka's Tamil Tiger rebels of continuing to forcibly recruit child soldiers, going against the permanent ceasefire agreement it signed with the government last month.

Under the terms of the agreement, both sides pledged to protect civilians from hostile acts such as intimidation, harassment and abduction.

Tamil Tigers deny the accusations

This latest charge against the rebels comes just after the United States government warned the Tigers they were putting the peace process in jeopardy.

Amnesty International says it knows of at least 18 children who have been recruited as soldiers by the Tamil Tigers in the northern district of Vavuniya.

This is on top of a list of 13 child-recruits the organisation released in mid-February.

According to Amnesty, some of the latest children were recruited well after the signing of the permanent ceasefire agreement last month.

However, the organisation says it is not yet clear if child recruitment will be considered a breach of the truce by the international monitoring committees.

US warning

Overall, Amnesty does concede that there are reports of fewer children being recruited in the north and east since the permanent ceasefire came into effect.

It also says a few children have been allowed to return home.

Young Tamils in refugee camps faced most risk

Amnesty International also says it has reports that the Tamil Tigers started their own investigations into the allegations of wrongdoing.

On Monday, the United States issued a statement saying it had credible reports the Tigers were engaged in child recruitment, extortion and arms smuggling.

In effect, they were jeopardising the peace process that has been widely welcomed by the international community.

The rebels' Chief Negotiator Anton Balasingham categorically denied the allegations.

He said elements opposed to peace were committing crimes in the name of the Tamil Tigers to discredit them.