Nuon Chea, left, who was the Khmer Rouge's chief ideologist and No. 2 leader, at the U.N.-backed war crimes tribunal last year. Pic: AP.

A U.N.-backed tribunal in Cambodia has sentenced two top Khmer Rouge leaders to life in prison.

Khieu Samphan, the regime’s head of state, and Nuon Chea, another senior leader, were found guilty of crimes against humanity in Phnom Penh, Thursday.  Defense lawyers said they would appeal the decision.

Judge Nil Nonn said the men were guilty of “extermination encompassing murder, political persecution, and other inhumane acts comprising forced transfer, enforced disappearances and attacks against human dignity”.

Both men, in dire health, denied any wrongdoing.

The charges centered on the forced exodus of millions of people from Cambodia’s cities and towns, and an execution site in the northwest where thousands of people were shot and buried in mass graves.

An estimated 1.7 million people died under the Khmer Rouge regime in the late 1970s.