"The leaders are aging ... that's why we have to start the process as soon as possible," said Michelle Lee, the UN's leading administrator to a planned tribunal of former top cadres of the regime.
Ieng Sary, who could be prosecuted for crimes committed during the communist regime's brutal rule over Cambodia, was admitted to hospital in Thailand with a serious heart condition, his son said on Friday.
"He was sent to hospital four days ago. He is very serious, otherwise he would not be sent to the hospital," Ieng Vuth told AFP from the former Khmer Rouge stronghold of Pailin in northwestern Cambodia.
Ieng Sary, 76, is one of 10 former top Khmer Rouge cadres who could stand trial in a genocide tribunal expected to start later this year.
He was a member of the inner circle of Pol Pot, the Khmer Rouge leader, who is blamed for orchestrating one of the worst genocides of the 20th century.
Two million people died during the
1975-79 rule of Khmer Rouge
As many as two million people died from starvation, overwork or execution during the 1975-79 rule of the Khmer Rouge, who erased all vestiges of modern life in their drive for an agrarian utopia.
So far, only two former regime leaders are in jail awaiting trial, and observers worry that others - including Nuon Chea, Pol Pot's number two, and Khieu Samphan, former Khmer Rouge head of state - could die before the joint UN-Cambodian tribunal is convened.
Lee said Cambodia still lacked $9.6 million in tribunal funds.
She said there was not a word yet whether donors would allow $6.9 million they contributed to a UN fund for Cambodia in the early 1990s to be channelled into the $56.3 million tribunal.