With four ageing Khmer Rouge leaders indicted for atrocity crimes and a free fifth leader in a Phnom Penh hospital, the US ambassador to Cambodia said Thursday delayed trials could mean a lost opportunity for justice.

"If we delay too much longer, all of them will be dead, and this would be very unfortunate, because you cannot try people who are already dead, it makes no sense," Ambassador Joseph Mussomeli said.

His statement comes just two days after Khieu Samphan, the nominal head of the regime who is widely expected to be indicted by the Khmer Rouge tribunal, suffered an apparent stroke in his home in Pailin. He was flown by government hospital for observation at Calmette Hospital on Wednesday.

Former Khmer Rouge leaders Ieng Sary, Ieng Thirith, Nuon Chea and Duch have all been charged and are awaiting trial on charges of atrocity crimes for their roles in the regime.

Khieu Samphan's apparent health deterioration revived concerns that ageing leaders might die before they see a day in court.

"The Cambodian people will loose a great opportunity" to try and punish those responsible for deaths under the regime if the leaders die, Mussomeli said.

Meanwhile, Youk Chhang, director of the Documentation Center of Cambodia, said Khieu Samphan's health condition could be a political ploy to avoid prosecution by the Khmer Rouge tribunal.

Tribunal spokesman Reach Sambath said the courts would not make any comment unless a warrant was issued for Khieu Samphan's arrest.