Cambodia's Khmer Rouge regime committed "genocide" during its reign of terror from 1975-1979, a UN-backed war crimes court said on Friday in a historic ruling.

"Brother Number Two" Nuon Chea, 92, and Khieu Samphan, 87, are last surviving senior leaders of the communist group that brutally ruled the Southeast Asian nation.

The tribunal judging their criminal responsibility for the deaths of an estimated 1.7 million Cambodians also found them guilty of committing crimes against humanity and other breaches of the Geneva Conventions.

"The chamber... finds that the crimes of genocide... were committed" against ethnic Vietnamese and Cham Muslims, presiding judge Nil Nonn said - the first time the court has issued such a ruling.

The large crowd of spectators attending the session included members of the Cham, a Muslim ethnic minority.

The leaders are already serving life sentences after being convicted in a previous 2011-2014 trial of crimes against humanity connected with forced transfers and disappearances of masses of people.

The Khmer Rouge sought to achieve an agrarian utopia by emptying the cities to establish vast rural communes. Instead their radical policies led to what has been termed "auto-genocide" through starvation, overwork and execution.

Treated as enemies

Lah Sath, a 72-year-old Cham man from eastern Kampong Cham province, brought his wife and four young granddaughters to the session. He said he often heard people talking about the trial and sometimes watched it on TV, but decided it was time to see it with his own eyes.

Just talking about the Khmer Rouge brought back horrible memories of life under their rule, he said. The Cham were treated as enemies and exploited without mercy as they were forced to do intensive farm labour, he recalled.

Lah Sath said his younger brother was killed by the Khmer Rouge for failing to take good care of a cow.

The tribunal has carried out just one other prosecution resulting in the 2010 conviction of Kaing Guek Eav, known as Duch, who as head of the Khmer Rouge prison system ran the infamous Tuol Sleng torture centre in the capital, Phnom Penh.

Prime Minister Hun Sen has declared he will allow no further case to go forward, claiming they would cause instability.

Hun Sen was a Khmer Rouge commander who defected and was later installed in government after the group was ousted from power by a Vietnamese invasion.

The tribunal has had its share of controversy after being set up in 2006 with more than $300m spent.

'Selective justice'

Theary Seng, a human rights lawyer and Khmer Rouge survivor whose parents were killed, told Al Jazeera she welcomed the verdict but questioned why only three people from the regime have been tried.

"The verdict is really the accumulation of years and years of waiting, we're talking about crimes committed over 40 years ago," she said.

"Justice is so selective and this is part of the problem with this court from the very beginning. To try three Khmer Rouge leaders when thousands had bloody hands is not comprehensible to anybody in the world, in particular the Cambodian victims. Our prime minister was former Khmer Rouge so he has every reason to interfere in the court."

Initial work had been done on two more cases involving four middle-ranking members of the Khmer Rouge, but they have been scuttled or bottled up by the tribunal, a hybrid court in which Cambodian prosecutors and judges are paired with international counterparts.

While the failure to have more extensive proceedings has discomforted some observers, others point to the tribunal's accomplishments

"International tribunals are better than the alternative, impunity. They will always be political and fall short of expectations," said Alexander Hinton, an anthropology professor at Rutgers University and author of two books about the tribunal.

The events covered by Friday's verdict span the four years of the regime of Pol Pot, Khmer Rouge's leader known as "Brother Number One."

Pol Pot - who fled to the jungle after Vietnam's invasion - was captured by a Khmer Rouge splinter group in 1997 and placed under house arrest. He died in his sleep a year later from heart failure.

Agencies