Khieu Samphan, the last surviving senior leader of Cambodia's Khmer Rouge regime, has failed in his appeal to overturn his genocide conviction.

The final verdict brings a close to a court process stretching 16 years, costing $500 million and resulting in the conviction of three of the Khmer Rouge's senior leaders.

The communist movement seized control of Cambodia in April 1975, unleashing a reign of terror lasting almost four years in the South-East Asian nation.

An estimated 2 million people died due to mass starvation, political persecution and mass killings, a catastrophic forced evacuation of the capital Phnom Penh, and the establishment of forced labour cooperatives in an effort to build a classless agrarian society.

Khieu Samphan, 91, was the head of state of what was then known as Democratic Kampuchea. He was a key public face of the regime, but denied having real decision-making power in his defence. 

He was put on trial alongside the Khmer Rouge's "Brother Number Two", chief ideologue Nuon Chea.

Both were convicted in two separate trials and handed life sentences in 2014 and 2018 for genocide, crimes against humanity and war crimes.

Nuon Chea died in 2019 at the age of 93.

The Khmer Rouge's key leader, Pol Pot, was never put on trial. He died in 1998.

Prison chief Kaing Guek Eav, known as Duch, was the only other leader to be convicted.

Duch oversaw the torture and forced confessions of thousands of prisoners at the notorious Tuol Sleng prison, who were later killed and buried at Cambodia's Killing Fields.

Genocide and forced marriages

Khieu Samphan's genocide conviction relates only to the treatment of one ethnic group – the Vietnamese.

But the genocide finding – often regarded as the crime of all crimes – has been viewed as a vindication of the collective suffering of the Cambodian people under the Khmer Rouge, according to Dr Rachel Killean, a senior lecturer at the University of Sydney Law School.

"Genocide is often locally framed as something that happened to the entire population, not only specific minority groups," she said. 

"Recognition of genocide can be symbolically meaningful for survivors regardless of the technical details surrounding legal judgements."

In its deliberations, the Supreme Court of the tribunal, formally known as the Extraordinary Chambers in the Courts of Cambodia (ECCC), described the Khmer Rouge's rule as "one of the most tragic and catastrophic periods in human history".

The proceedings also addressed the religious persecution of Cham Muslims — who were forced to eat pork, had their Korans burned and mosques dismantled — and Buddhist monks — who were referred to "worms" or "leeches" — as well crimes of forced marriage and forced sexual intercourse.

Dr Rosemary Grey, also from the University of Sydney Law School and the Sydney South-East Asia Centre, said there was legitimate criticism of the court's failure to prosecute some sexualised crimes during the Khmer Rouge period, including forced pregnancy and rapes that were not deemed to be part of official policy. 

But she said an important part of its legacy was bringing to light forced marriages orchestrated by the regime.

Dr Grey pointed out the 2018 trial judgement "controversially drew a distinction between the men's and women's experience of forced sexual intercourse within forced marriages". 

"It found that the women's experience of sexual violence was grave enough to amount to the crime against humanity of 'other inhumane acts', but the men's experience was not," she said. 

But the appeals chamber reversed that finding on Thursday.

"The victims of forced sexual intercourse comprise both female and male victims," the Supreme Court said, adding the practice was part of the party's attempts at population control.