Phnom Penh - Judges at Cambodia's UN-backed Khmer Rouge tribunal on Thursday postponed a hearing for the fallen regime's ailing former foreign minister after doctors declared him too ill to attend court.

Ieng Sary, 83, was due in court to appeal an order to extend his pre-trial detention but an early-morning health check found him unfit to attend the session.

Prosecutors and defence lawyers argued over whether to hold the hearing in his absence, but judges decided to adjourn the hearing to April 1.

The former schoolteacher suffers from heart disease and high blood pressure and was rushed to hospital on Monday night after blood was found in his urine.

It was his ninth hospitalization since he was arrested in November 2007.

He returned to the tribunal's detention facility on Wednesday afternoon.

Ieng Sary's lawyers argued for his release, saying his provisional detention was jeopardizing his physical and mental health.

"Provisional detention should not be used as punishment for alleged crimes, but there is indication it might be used in this way in this case," international co-defence lawyer Michael Karnavas said at a press conference after the hearing.

Ieng Sary is one of five former Khmer Rouge leaders facing trial for their roles in the deaths of up to 2 million people through execution, starvation or overwork during the radical group's 1975-79 reign of terror in Cambodia.

His wife, Ieng Thirith, is among the detainees.

The first stage of the trial of former Tuol Sleng torture facility chairman Kaing Guek Eav, known by his revolutionary name Duch, was held last week.

The Khmer Rouge tribunal announced on Monday that Duch's trial would resume on March 30.

He is the first detainee to be indicted for crimes against humanity. (dpa)