Cambodia's Khmer Rouge chief ideologist and 'Brother Number Two' Nuon Chea died on Sunday at the age of 93, a court spokesman has confirmed.

Key points: Between 1.7 and 2.2 million people died during the 1975 to 1979 rule of the Khmer Rouge

Between 1.7 and 2.2 million people died during the 1975 to 1979 rule of the Khmer Rouge Most of the victims died of starvation, torture, executions, exhaustion or disease

Most of the victims died of starvation, torture, executions, exhaustion or disease Former president Khieu Samphan and Kaing Guek Eav are also serving life sentences

A UN-backed court found Nuon Chea guilty of crimes against humanity in 2014, and a further trial in 2018 found him guilty of genocide.

Nuon Chea was sentenced to life in prison last year, almost four decades after the Maoist regime that oversaw the 'Killing Fields' was overthrown.

Khmer Rouge Tribunal spokesman Neth Pheaktra confirmed to the ABC that Nuon Chea died Sunday evening in a Phnom Penh hospital.

"He had been transferred from the Extraordinary Chambers in the Court of Cambodia's (ECCC) detention centre to the Khmer-Soviet Friendship hospital for medical treatment since 2 July," Mr Neth said.

"Nuon Chea's family has been informed."

Researchers believe Nuon Chea was responsible for the murderous policies of the Khmer Rouge. ( Tuol Sleng Genocide Museum )

Nuon Chea was among a small clique — led by 'Brother Number One', Pol Pot — of mostly French-educated communists who rose to lead a bloody revolution against a US-backed government after their country was engulfed by the Vietnam War.

The extremist ideology of the 1975-79 regime sought to turn Cambodia back to "year zero" in its quest for a peasant utopia.

Researchers believe Nuon Chea was responsible for the murderous policies of the Khmer Rouge and was directly involved in its purges and executions.

Most of the victims of the regime died of starvation, torture, exhaustion or disease in labour camps or were bludgeoned to death during mass executions.

Between 1.7 and 2.2 million people, almost a quarter of Cambodia's population, died during the almost four-year rule of the Khmer Rouge.

Currently, only former president Khieu Samphan and Kaing Guek Eav, also known as Comrade Duch, are serving life sentences after being convicted of genocide and crimes against humanity.

Nuon Chea and Khieu Samphan were both found guilty of genocide. ( Supplied: ECCC )

'He outlived Cambodian life expectancy by 25 years'

Nuon Chea was deputy secretary of the Communist Party of Kampuchea, a member of the Standing Committee and Central Committee, and chairman of the Democratic Kampuchea People's Assembly.

He was arrested along with four other party leaders in 2007 and was on trial on charges of genocide, crimes against humanity and grave breaches of the 1949 Geneva Convention.

Two of those defendants — Ieng Sary and his wife Ieng Thirith, who was found to be mentally unfit to stand trial — have also passed away during the course of the 12-year trial.

Nuon Chea pled not guilty to all charges and an appeal against the charge of genocide was underway at the time of his death.

"He has been committed to seeing the proceedings through. He has been fighting to stay as fit as he could," Nuon Chea's lawyer Doreen Chen told the ABC.

"He has already outlived Cambodian life expectancy for men by 25 years."

Public records show while his health has been in decline for years, his mental state was still sound.

Regular examinations determined him to be at full capacity to participate in the trail.

'12 years, $300 million, no judgement'

The UN-backed tribunal is judging the criminal responsibility of former Khmer Rouge leaders. ( AP: Heng Sinith )

Ms Chan said Nuon Chea's appeal against the second charges of genocide were filed on July 1, but his death has left those proceedings in limbo.

"The question is what happens to Nuon Chea's case given that he has passed away now, and legally the position on that is actually unclear," she said.

If a defendant dies on trial, the case ceases immediately, but there is nothing in Cambodian law to determine what happens in the case of an appeal.

"What seems clear is that legally, in fact, because he died on appeal the trial judgement against him is effectively vacated," Ms Chan said.

"So 12 years, $300 million, no judgement."

Ms Chan added that it is possible his appeal may go forward, but court spokesman Mr Neth told the ABC, "According to the court proceeding, his case will be terminated".

'He can't escape his punishment from God'

Nuon Chea's defence was not a denial of what happened during the period. ( Supplied: Documentation Center of Cambodia Archives )

Until his arrest in 2007, Nuon Chea lived as a private citizen in Prum, a small town on the Thai-Cambodian border.

He spoke openly about the inner workings of the party and his role in Cambodia's Killing Fields to Cambodian reporter Thet Sambath, which was revealed in the documentary Enemies of the People.

Ms Chan said Nuon Chea's defence was not a denial of what happened during the period.

The Khmer Rouge regime Pol Pot's ultra-Maoist Khmer Rouge guerrillas launched a bloody agrarian revolution from 1975 until 1979

Pol Pot's ultra-Maoist Khmer Rouge guerrillas launched a bloody agrarian revolution from 1975 until 1979 At least 1.7 million people — 21 per cent of the population — were executed or died of disease, starvation or overwork

At least 1.7 million people — 21 per cent of the population — were executed or died of disease, starvation or overwork Almost every Cambodian alive lost a family member under the Khmer Rouge

"Nuon Chea has always accepted moral responsibility for the events of democratic Kampuchea. That's never been in doubt," Ms Chan said.

"He said his participation in the tribunal had been to clarify the truth of what happened … to look at the roles played by foreign countries in particular Vietnam and the US among others."

Youk Chhang, director of Documentation Centre of Cambodia, an organisation which provided thousands of documents as evidence to the UN-backed court which prosecuted the Khmer Rouge, said Nuon Chea had escaped his life sentence by dying.

"He was born just like the rest of us, but became driven by power and prejudice against his own people," Mr Youk said.

"He can escape justice from the court of law, but he can't escape his punishment from God."