Nuon Chea, a top leader of the Khmer Rouge, reported himself ill on Friday, raising the question among observers whether a tribunal will take place in time.
In a brief phone conversation with VOA Khmer, Nuon Chea, 81, Pol Pot's chief lieutenant, said he had fallen ill.
Nuon Chea declined to comment on the progress of the tribunal, which began moving forward again last week when international and Cambodian judges agreed on a set of rules governing the tribunal.
"Excuse me, I am sick," Nuon Chea said, before hanging up.
His announcement raised new concerns that a tribunal had stalled too long and that the architects of Cambodia's genocide would escape justice. Former leaders of the regime have become increasingly feeble, and the tribunal will likely not begin trials until 2008.
Nuon Chea suffers from high blood pressure and heart disease and sometimes walks with a cane.
A tribunal spokesman told VOA Khmer that judges and prosecutors were aware potential defendants and witnesses were aging and that recent breakthroughs in tribunal discussions meant trials will definitely take place.
"Everybody thinks that the Khmer Rouge trial will start next year, because they know the former Khmer Rouge leaders are getting old, and the witnesses are also old," spokesman Reach Sambath said. "We must proceed rapidly, and I want to clarify that the work we are doing is a sign to the world and other leaders that there will be a trial."
Youk Chhang, director of the Document Center of Cambodia, said the courts should work hard to make sure the trials can take place.
The tribunal jurists "should spend the time even though it is limited, in time for the aging Khmer Rouge leaders," he said.