Caroline Gluck in Phnom Penh: "Ambiguous signals from government"

Khieu Sampan and Nuon Chea are seeking assurances that they will not stand trial, according to the Reuters news agency quoting military sources in the western Cambodian town of Pailin.

"These people are elderly men. They won't live much longer than 20 years - why not let them live peacefully for the rest of their lives?" said a senior military source.

In his first reaction to their surrender, the Cambodian leader, Hun Sen, hinted that the two might be pardoned.

His comments were echoed by Cambodia's monarch, King Sihanouk, who expressed his deep gratitude that the men had surrendered.

Call for a trial

However, in a strongly-worded statement, the Human Rights Watch Asia group said there could be no national reconciliation in Cambodia until Khmer Rouge leaders faced an international tribunal.

Opposition leader Sam Rainsy: "Justice must be above politics"

The group's director, Sidney Jones, said it was unthinkable to allow the men simply to return to society as if one of the worst massacres of the 20th century never took place.

The BBC's Caroline Gluck, reporting from Phnom Penh, says the government is giving mixed signals as one of Hun Sen's senior aides has said the government still backs the idea of a tribunal.

Key figures

Khieu Samphan was the group's nominal leader in the 1980s, while Nuon Chea was a shadowy figure who devised much of the Khmer Rouge's extreme left-wing ideology.

Their capture means the only senior Khmer Rouge commander still on the loose is Ta Mok, a lieutenant of the reviled dictator Pol Pot, who died of a heart attack in March.

Khieu Samphan: Public face of the Khmer Rouge

Khieu Samphan became known as the public face of the secretive Khmer Rouge when he represented the movement during 1991 peace talks.

He was officially head of state in Cambodia between April 1976 and the overthrow of the guerrillas by the neighbouring Vietnamese in 1979.

In that period - later dubbed the Killing Fields after the Oscar-winning film - an estimated 1.7 million Cambodians died through disease, starvation or execution.

Nuon Chea, 71, was the official number two to Pol Pot.

The pair's surrender comes two years after the defection of top Khmer Rouge cadre Ieng Sary, which coincided with a fatal split in the movement.

The last Khmer Rouge fighters surrendered to the government earlier this month.