The London-based group called for United Nations agencies and aid groups to be given immediate access to Hmong rebels inside Laos following the attack, which it said also included the rape of four of the victims.

Amnesty said it had gathered evidence "including video and witness testimony of an attack by Lao soldiers against a group of five children, four of them girls, in the Xaisomboune military zone on 19 May 2004."

"The attacks violate the most fundamental principles of international human rights and humanitarian law. These rapes and killings constitute war crimes," Amnesty said in a statement.

"The Lao authorities must bring to justice those responsible for this atrocity and cease attacks on unarmed civilians."

A foreign ministry spokesman in the Lao capital Vientiane, Yong Chanthalangsy, dismissed the Amnesty report as "false allegations aiming at creating disorder."

Asked whether the government would open an investigation, he said: "Of course, we try to verify these kind of allegations. But we can't run constantly after those who want to tarnish the reputation of the country with such fabrications."

Small bands of ethnic minority rebels, predominantly Hmong, are a hangover from the CIA-sponsored guerrilla army mobilized to wage Washington's secret campaign in Laos during the Vietnam War.

Earlier this year, hundreds of them left their forest hideouts in the north of the country to surrender.

The Lao military has been accused by human rights groups of using heavy-handed and often brutal tactics to eliminate the rebels. But Vientiane has never even acknowledged their existence.

According to Amnesty, the unarmed children, aged between 13 and 16, were mutilated before being killed. Four of the five were girls and were also raped. The assault was carried out by a group of approximately 30-40 soldiers.

The Amnesty statement quoted a witness who has subsequently fled Laos as saying one girl was shot in each breast and that another girl was disembowelled. Several other people managed to escape the attack.

"The Lao authorities must, as a matter of utmost urgency, permit UN agencies and independent monitors unfettered access to those rebels," Amnesty said.

"They must also permit humanitarian agencies to provide medical and food assistance to those injured as a result of this and other military actions against the rebels."

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