Two leaders of Cambodia's former Khmer Rouge regime have appealed 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.

The two men are the regime's only surviving senior leaders. In August, they were convicted of crimes against humanity in a ruling 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 tribunal said in a statement that the former leaders insisted they are innocent and filed hundreds of grounds for appeal.