The tribunal's chief judge, Nil Nonn, opened the hearings on Wednesday marking the start of the second phase of the war crimes trial of two Khmer Rouge leaders - Nuon Chea, 88 and Khieu Samphan, 83.

Due to the defendants advanced age and poor health, the case against them was divided into separate smaller trials.

During Wednesday's proceedings, Nuon Chea, known as Khmer Rouge leader Pol Pot's "Brother Number 2," remained in his holding cell because he is unable to sit for long periods of time. Khieu Samphan, former head of state, appeared in court and took notes.

The hybrid national and United Nations tribunal was set up in 2006 to try the worst crimes that occurred under Pol Pot's ultra-Marxist regime between 1975 and 1979. An estimated 1.7 million Cambodians died from starvation, exhaustion, disease or execution during his rule.

Since the first trial began, one person has been prosecuted. The head of the S-21 torture center, Kaing Guek Eav - alias Duch - received life imprisonment for overseeing the deaths of 15,000 people.

A verdict in the first phase is due on August 7, however observers and victims have raised concerns that the ageing Khmer Rouge leaders may not survive to see the end of the trials.

Former regime foreign minister Ieng Sary died aged 87 last year while on trial for war crimes, while his wife was freed from jail in September 2012 after being ruled unfit for trial due to dementia.

hc/dr (AP, AFP, dpa)