Judges at Cambodia's Khmer Rouge war crimes tribunal suspended proceedings on Monday as the oldest of the three former regime leaders on trial remained in hospital suffering from bronchitis.

Ieng Sary, 86, was rushed to hospital with breathing difficulties on Thursday afternoon. The hearing continued without him until the court, which does not sit on Fridays, adjourned for the weekend.

Ieng Sary is the most frail of the trio in the dock for their roles in the deaths of up to two million people in the late 1970s.

He will be discharged on Tuesday at the earliest, according to a doctor's report read out in court Monday which said the regime's former foreign minister had "strong bronchitis, which comes on top of existing cardiovascular issues".

The UN-backed court will reconvene on Wednesday when judges will assess his condition, presiding judge Nil Nonn said.

Health fears have long hung over the tribunal with the octogenarian defendants all suffering from varying ailments, but this marks the first time one of them has been taken ill since their trial opened in November.

Lawyers for Ieng Sary told the court their client had not waived his right to be present at the trial and said proceedings should not continue without him.

"It's regrettable that Mr Ieng Sary is ill," said defence lawyer Michael Karnavas, adding that his client is entitled to help with his own defence case.

"We cannot get instructions from Mr Ieng Sary if he is not present or at least following the proceedings by videolink," he said. "We will have to wait until Mr Ieng Sary's health is better."

Ieng Sary and his co-defendants -- "Brother Number Two" Nuon Chea and former head of state Khieu Samphan -- deny charges of war crimes, crimes against humanity and genocide.

Known as one of the few international faces of the secretive Khmer Rouge regime, Ieng Sary has exercised his right to remain silent during the trial.

The 1975-1979 regime oversaw one of the worst horrors of the 20th century, wiping out nearly a quarter of the population through starvation, overwork and execution in a bid to forge a communist utopia.