1 of 2
Heng Sinith, Associated Press
Cambodian villagers wait to enter a courtroom before the first appeal hearings against two former Khmer Rouge senior leaders, Khieu Samphan and Nuon Chea, at the U.N.-backed war crimes tribunal in Phnom Penh, Cambodia, Thursday, July 2, 2015. The two former Khmer Rouge leaders are facing charges of genocide, war crimes and crimes against humanity.

PHNOM PENH, Cambodia — The two surviving senior leaders of Cambodia's former Khmer Rouge regime launched appeals Thursday of their convictions by a U.N.-backed tribunal that sentenced them to life in prison for their roles during the group's brutal rule in the 1970s.

Khieu Samphan, the Khmer Rouge's 83-year-old former head of state, and Nuon Chea, its 88-year-old chief ideologue, appeared in court Thursday for the first day of the appeals. The tribunal said a ruling on the appeals by its Supreme Court is not expected until early next year.

The two men, both frail and ailing, are the regime's only surviving leaders. The case against them was divided into separate smaller trials in an effort to render justice before they die. They are currently facing genocide charges in a second trial that is expected to take years to complete.

Their appeals are against the rulings in their first trial that ended last August with convictions on charges of crimes against humanity. They came three and a half decades after the Khmer Rouge's 1975-1979 rule, which left an estimated 1.7 million people dead from starvation, disease and execution. The group's top leader, Pol Pot, died in 1998.

The tribunal said in a statement that Khieu Samphan and Nuon Chea insisted they are innocent of crimes against humanity and filed 223 and 148 grounds for appeal, respectively.

The case focused on just one of the many mass killing sites and the forced exodus of millions of people from Cambodia's towns and cities, where even hospitals were emptied of patients.

During the initial trial, Khieu Samphan acknowledged that mass killings took place. But testifying before the court in 2011, he said he was just a figurehead with no real authority. He denied ordering any executions himself, calling the allegations a "fairy tale." Instead, he blamed Pol Pot for the group's extreme policies.

Nuon Chea, who is known as Brother No. 2 for being Pol Pot's trusted deputy, also denied responsibility, testifying in 2011 that Vietnamese forces — not the Khmer Rouge — had killed Cambodians en masse. "I don't want them to believe the Khmer Rouge are bad people, are criminals," he said of those observing the trial. "Nothing is true about that."