A U.N.-backed tribunal on Wednesday began a hearing to prepare for the genocide trial of the two senior surviving leaders of Cambodia's Khmer Rouge, under whose rule an estimated 1.7 million people died in the late 1970s from starvation, exhaustion, disease and execution.

Khieu Samphan, the regime's head of state, and Nuon Chea, right-hand man to the group's late leader, Pol Pot, are already set to face sentencing next week after being tried for crimes against humanity related mostly to the communist group's forced movement of millions to the countryside when it took power in 1975.

The tribunal's chief judge, Nil Non, opened the hearings by reading the charges and crime sites set to be heard in the newest trial segment.

Tribunal officials say their second trial, with witnesses and the presentation of evidence, is likely to begin in the last quarter of this year. It will cover additional crimes against humanity, and add charges of genocide for the killings of members of Cambodia's Vietnamese and Cham ethnic minorities.

The crimes of rape and forced marriages will also be considered for the first time by the tribunal.

"The purpose of today's hearing is to clarify issues ahead of the case," Non said.

Because he is unable to sit for long periods of time, Nuon Chea remained in his holding cell. Khieu Samphan appeared in good health, diligently taking notes as he sat in court.

This week's initial hearing will cover technical matters such as witness lists and procedural objections by the contending parties.

Because of the advanced age and poor health of the defendants, the case against them was divided into separate smaller trials. But some critics feel that convictions on lesser charges may be an affront to history.

"The goal (of breaking the case into two parts) was to have a shortened process to reach a verdict so that the victims would have an opportunity to see justice before the elderly accused passed away," said Anne Heindel, co-author of "Hybrid Justice: The Extraordinary Chambers in the Courts of Cambodia."

"The problem that they're facing is trying to come up with an expedited verdict as opposed to having a comprehensive narrative."

The first trial, which began in November 2011 and lasted two years, focused solely on forced evacuations and a mass execution of soldiers who had fought against the Khmer Rouge during a bitter 1970-75 civil war.

That trial, a verdict for which is expected Aug. 7, was repeatedly hampered by delays, and by the end, it had lost half its original defendants.

Khmer Rouge Foreign Minister Ieng Sary died in March 2013, while his wife, Social Affairs Minister Ieng Thirith, was deemed unfit to stand trial due to dementia.

Nuon Chea, 88, and Khieu Samphan, 83, required occasional hospitalization, also slowing the proceedings.

"The court should speed up their work and finish their duty before the two defendants die," said Svay Sophoan, 55, who lost his father and several relatives during the 1975-79 Khmer Rouge reign of terror. "I need justice, a justice that comes from a fair trial because these Khmer Rouge leaders have killed millions of people, not taken just two or three lives."

So far, only one person has been convicted by the tribunal since it began in 2006. The head of the notorious S-21 torture center, Kaing Guek Eav — also known as Duch — received a sentence of life imprisonment in February 2012.

----

Associated Press Writer Sopheng Cheang contributed to this report.