Cambodia's Khmer Rouge tribunal restarted genocide hearings Thursday against the former regime's most senior surviving leaders, with the first witness being called to testify against the ailing octuagenarians.

Proceedings had been postponed since November after defense lawyers threatened a boycott because they said they were still working to appeal an earlier verdict.

Khieu Samphan, the 1970s regime's head of state, and Nuon Chea, a right-hand man to communist group's late leader, Pol Pot, were sentenced to life in prison in August after being found guilty of crimes against humanity. They are now on trial on separate charges of genocide against minorities, and rape and forces marriages — the first time such accusations have been put to trial.

Some 1.7 million people are estimated to have died from starvation, disease and execution due to the group's extremist policies, and there is growing concern that the two men could die before the genocide trial can be completed.

Thursday's hearing ended early, at midday, because Khieu Samphan, 83, was dizzy and suffering from high blood pressure. Nuon Chea, 88, did not appear in court and watched the proceedings via a video link from a holding cell due to his poor health.

The witness who testified on Thursday, 55-year-old Meas Sokha, was one of several dozen survivors who are expected to give their accounts before the court.

Responding to questions from prosecutors, Sokha said 12 of his relatives, including his parents, were arrested. He said several of them were executed by authorities.