PHNOM PENH (AFP) -- A rights group Sunday called on Cambodia and the United Nations to provide better medical care to former Khmer Rouge military leader Ta Mok, who has been in a coma for the past few days. The Cambodian Human Rights Action Committee said Ta Mok, 80, could die without ever going on trial for genocide, and the loss of the much-feared military chief would affect the UN-Cambodian Khmer Rouge tribunal "negatively."

"The committee wishes to call for both the Cambodian government and the United Nations to pay stronger attention to Ta Mok's health by ensuring that he immediately will be sent to the best hospital for efficient care," it said.

Ta Mok, known as "The Butcher" for his bloody purges during the 1975-1979 Khmer Rouge rule, was taken to a military hospital in Phnom Penh on June 29 due to heart, lung and respiratory problems.

But his condition has worsened since then and the one-legged former commander has been in a coma for the past few days, according to his niece Ven Dara. The committee, an umbrella group of more than 20 rights organizations, called Ta Mok "one of the most potential persons among the most responsible Khmer Rouge leaders."

Ven Dara has asked the government to transfer her uncle to neighboring Thailand for better medical care. It has yet to reply to her request.

Cambodia's long-awaited tribunal to try former Khmer Rouge leaders finally got under way this month, 27 years after the fall of the ultra-Maoist regime blamed for the deaths of up to two million people.

Prosecutors began their investigations but the first trials are not expected until mid-2007.

Up to two million people were executed or died of starvation and overwork between 1975 and 1979, when the communist Khmer Rouge forced millions into the countryside in their drive for an agrarian utopia.

Ta Mok was arrested in March 1999 and has been held in a military prison ever since. He is one of only two surviving former Khmer Rouge leaders in detention.

The other is Kaing Khek Iev, known as Duch, who headed the Khmer Rouge's notorious S-21 prison, where some 17,000 men, women and children were tortured before being executed. Only seven people survived the death camp.

Ta Mok ousted Khmer Rouge leader Pol Pot in 1997 and was the group's final leader. Pol Pot died in 1998.

Others -- including Pol Pot's top deputy Nuon Chea, former head of state Khieu Samphan and ex-foreign minister Ieng Sary -- live freely in Cambodia.

But all are elderly and suffer from poor health, raising fears that they might die before the start of the trials.