The UN-backed war crimes tribunal in Cambodia has opened the trial of four top leaders of the Khmer Rouge accused of genocide, crimes against humanity, religious persecution, torture and other crimes in the late 1970s.

The elderly defendants, including "Brother Number Two" Nuon Chea and former head of state Khieu Samphan, looked frail as they sat in the dock on Monday.

"The (court) opens the initial hearing of case 002," chief judge Nil Nonn told the court before proceeding to list the charges against the accused.

In its first trial, the tribunal sentenced former prison chief Kaing Guek Eav to 30 years in jail last July for overseeing the deaths of 15,000 people.