(CNN) Almost four decades on from the collapse of Pol Pot's tyrannical communist regime, an international tribunal has ruled that the Khmer Rouge committed genocide, a landmark verdict that is hoped will bring closure to millions of Cambodians.

More than 1.7 million people, or approximately a fifth of Cambodia's population, are believed to have died from forced labor, starvation and execution under the Khmer Rouge which ruled the country between 1975 and 1979.

On Friday, the regime's two most senior surviving members , Nuon Chea and Khieu Samphan, now 92 and 87, were found by the court to be guilty of genocide of Vietnamese in Cambodia between 1975 and 1979.

Nuon Chea was also found guilty of genocide against the Cham ethnic group in Cambodia during that time.

In addition, the pair were found guilty of murder, extermination, deportation, enslavement, imprisonment, torture, persecution on political, religious and racial grounds, and other inhumane acts.

Both men were sentenced to life in prison.

Nuon Chea, known in the regime as "Brother Number Two," and Khieu Samphan known as "Brother Number Four," are already serving life sentences in Cambodia for crimes against humanity. Friday's sentences will be merged with their existing sentences.

Phil Robertson, Deputy Asia Director for Human Rights Watch, told CNN the conviction should be celebrated.

"This has been a long time coming, and obviously there's been a tremendous amount of time taken. But you know this conviction is hugely important and I think no one should discount the importance of this action today," said Robertson.

"(To prove) the intent of the Khmer Rouge was to wipe these people out would be something that required a lot of time and a lot of expertise. It required a deep dive into Khmer Rouge archives. And so the importance I think was that they get this verdict right, and that it was based on incontrovertible evidence."

Former senior Khmer Rouge leaders Khieu Samphan and Nuon Chea pcitured in Phnom Penh in 2011.

UN-backed court

Friday's decision was delivered by the Extraordinary Chambers in the Courts of Cambodia (ECCC) which is based in the Cambodian capital of Phnom Penh. The court is a special United Nations-backed tribunal that was formed in 2006 to prosecute senior Khmer Rouge leaders and other regime figures.

Prior to Friday's convictions, the ECCC has delivered only three verdicts.

In addition to 2014 case which found Nuon Chea and Khieu Samphan guilty of crimes against humanity, the court also sentenced Kaing Guek Eav, commonly known by his alias, Duch, to life imprisonment in 2010, for war crimes, crimes against humanity, murder and torture.

Kaing Guek Eav was the commandant of the notorious Tuol Sleng S-21 prison in Phnom Penh, where more than 14,000 people died.

Photos: Cambodia's bloody past Cambodia's bloody past – Cambodian and international journalists watch a live video feed showing the verdicts in the trial of former Khmer Rouge leader "Brother Number Two," Nuon Chea, and former Khmer Rouge head of state Khieu Samphan, August 7, 2014. Hide Caption 1 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – A Cambodian man Khieu Samphan on a television during the trial at the Extraordinary Chamber in the Courts of Cambodia (ECCC) in Phnom Penh on August 7. He and Nuon Chea were found guilty of crimes against humanity and sentenced to life in prison. Hide Caption 2 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – Known as Brother Number Two, Nuon Chea was considered Khmer Rouge leader Pol Pot's right hand man. This is an image from court in 2011.

Hide Caption 3 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – As the former head of state for the Khmer Rouge, Khieu Samphan occupied a number of key roles as the government tortured, starved and killed its people. Hide Caption 4 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – This undated photo, which may have been taken in 1989, shows Pol Pot, the former leader of the Khmer Rouge. He was under house arrest when he died in 1998 and never faced charges for the slaughter under his reign. Hide Caption 5 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – At least 1.7 million people were killed under the Khmer Rouge's brutal regime, which controlled Cambodia from 1975 to 1979. This photo shows open mass graves in 1979. Hide Caption 6 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – Khmer Rouge guerilla soldiers wearing black uniforms drive into Phnom Penh in April 1975, as Cambodia falls under the control of the Khmer Rouge. Hide Caption 7 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – A woman cries next to a dead body in April 1975 in Phnom Penh, after the Khmer Rouge enter the Cambodian capital and establish the government of Democratic Kampuchea (DK). Hide Caption 8 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – Historical, undated photo of Nuon Chea. He held a number of positions during the regime's rule, including a short stint as acting prime minister. Hide Caption 9 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – Historical, undated photo of Khieu Samphan. During his trial, Khieu Samphan expressed remorse, claiming he was unaware of the full extent of the atrocities. He became the public face of the Khmer Rouge as it sought international credibility after its fall. Hide Caption 10 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – A Cambodian man sits in Choeung Ek Killing Fields near a tree that was used to beat children to death during the Khmer Rouge regime on August 6, 2014 in Phnom Penh, Cambodia. Hide Caption 11 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – A Cambodian man stands by a wall of photographs of prisoners of the Khmer Rouge regime in one of the rooms of Tuol Sleng prison, also known as S-21, on August 6 in Phnom Penh. Hide Caption 12 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – Cambodian citizen Sam Vishna looks at a collection of skulls that make up a map of Cambodia at Tuol Sleng Prison Museum in Phnom Penh in 1998. Hide Caption 13 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – Cambodian Buddhist monks bless victims' skulls at the Choeung Ek Killing Fields memorial in Phnom Penh on April 17, 2008. Hide Caption 14 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – Cambodian Sao Phen prepares skulls and bones of victims of the Khmer Rouge inside a stupa in Kandal province in 2009. Hide Caption 15 of 16 Photos: Cambodia's bloody past Cambodia's bloody past – A Cambodian woman looks at portraits of victims of the Khmer Rouge at the Tuol Sleng genocide museum in Phnom Penh on November 17, 2011. Hide Caption 16 of 16

The ECCC's glacial pace in securing only three convictions in more than a decade, at a cost of more than $260 million, has been widely criticized, especially given the suspects were nearing the end of their natural lives.

However, former ECCC investigator Craig Etcheson previously told CNN that public opinion surveys had consistently shown that a large majority of the Cambodian public supported the ECCC process.

"I would suggest that with upwards of two million people killed during the Khmer Rouge regime, a $200 million dollar tribunal works out to roughly $100 per murder victim. Surely that is not an excessive price to seek justice for such a monumental crime," he said.

A Cambodian man sits in Choeung Ek Killing Fields near a tree that was used to beat children to death under the Khmer Rouge regime, on August 6, 2014 in Phnom Penh, Cambodia.

Pol Pot's right-hand man

Nuon Chea, born in 1926, was Khmer Rouge leader Pol Pot's brother-in-law, and was considered his right-hand man and a key ideologist throughout the regime's reign of terror.

Trained in law in Bangkok, the 88-year-old was second-ranked in the Communist Party of Kampuchea (as the Khmer Rouge is officially known) and served a short stint as Democratic Kampuchea's prime minister.

During a his trial in 2014, prosecutors at the Courts of Cambodia (ECCC) described him as an extremist who "crossed the line from revolutionary to war criminal, responsible for the deaths of hundreds of thousands of Cambodians."

Following the collapse of Democratic Kampuchea in 1979, he remained a leading Khmer Rouge figure in the years the movement operated as a rebel guerrilla force in Cambodia's west.

He surrendered in 1998, striking a deal with the government that allowed him to live as a free man near the Thai border until his arrest in 2007, according to the ECCC.

In his final statement to the court, Nuon Chea admitted he carried "moral responsibility" for events during the period, but also affirmed his innocence, according to the ECCC.

"The CPK's policy and plan were solely designed to one purpose only, to liberate the country from the colonization, imperialism, exploitation, extreme poverty and invasion from neighboring countries," he said.

"The CPK's policy was clear and specific: it wanted to create an equal society where people were the master of the country ... The CPK's movement was not designed to kill people or destroy the country. My hope and wishes were betrayed by those who destroyed the movement."

Like many other Khmer Rouge leaders, Khieu Samphan studied in Paris, publishing his doctoral dissertation on "Cambodia's economy and industrial development." On his return home, he became a professor and then took on a senior government position before joining the Khmer Rouge rebels.

In 1976, he became the head of state of Democratic Kampuchea, and in 1987, years after the fall of Democratic Kampuchea, he replaced Pol Pot as the head of the Khmer Rouge after the former's retirement.

Throughout the previous 2014 trial, he expressed remorse for the suffering of victims, at one point offering Buddhist prayers for the souls of those who had died. But he repeatedly expressed his position that he was merely a figurehead, with no role in Khmer Rouge policy.