Two top Khmer Rouge leaders have been sentenced to life in prison by a UN-backed tribunal in Cambodia.

The historic verdicts were announced against Khieu Samphan, the Khmer Rouge regime's (83), and Nuon Chea, its chief ideologue (88).

The charges centred 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.

Nuon Chea (L) and Khieu Samphan remained impassive as they were sentenced | Image: Sky News

The Khmer Rouge was the name given to followers of the Communist Party of Cambodia, which ruled the country from 1975 to 1979.

Led by Pol Pot, the regime's rule led to the deaths of more than two million Cambodians who were starved, tortured and executed.

Nuon Chea, wearing his trademark sunglasses, sat in a wheelchair in the dock as the verdict was read, while Khieu Samphan stood next to him.

Stony response

The men showed no emotion as they were sentenced following the two-year trial, though survivors of the regime cried and applauded as the men were sentenced.

"This is the justice that I have been waiting for these last 35 years," said survivor Khieu Pheatarak (70), one of a few dozen survivors at the Phnom Penh-based court to hear the verdict.

"I will never forget the suffering but this is a great relief for me. It is a victory and an historic day for all Cambodians," she said.

She was among tens of thousands of Cambodians taken from their homes at gunpoint in 1975 by the Khmer Rouge's peasant army and forced into agricultural work in an attempt by the regime to create a totally self-sufficient and classless society.

Pair will appeal

The men's lawyers said they would appeal the verdict. "It is unjust for my client. He did not know or commit many of these crimes," Son Arun, a lawyer for Nuon Chea, told reporters.

Despite both defendants denying any knowledge of the Khmer Rouge's crimes at the time, they both eventually expressed remorse for the suffering inflicted on the Cambodian people by the regime.

But the trial's judge said the gravity of the crimes meant they would "remain in detention until this judgement becomes final".

Nil Nonn said the pair were "guilty of crimes against humanity, of extermination... political persecution, and other inhumane acts".

Despite the verdict, it is feared the ageing men will not live long enough to serve much of their sentence.

Former foreign minister Ieng Sary died aged 87 last year while still on trial on charges of genocide. His wife Ieng Thirith was released in 2012 after it was ruled she was too ill to stand trial.