By Prak Chan Thul
PHNOM PENH (Reuters) - A U.N.-backed tribunal in Cambodia sentenced the top two surviving cadres of the Khmer Rouge regime to life in jail on Thursday after finding them guilty of crimes against humanity for their part in the 1970s "killing fields" revolution.
"Brother Number Two" Nuon Chea, 88, and former President Khieu Samphan, 83, were complicit in forced evacuations, murders and executions orchestrated by the regime as part of its ultra-Maoist revolution, judge Nil Nonn said.
The verdict was only the second delivered in the tribunal's nine years of operation. The two men will remain in court to face separate charges of genocide in a second phase of the complex trial that started last week.
The judge, taking an hour and 20 minutes to read out the verdict, said their crimes included "extermination encompassing murder, political persecution and other inhuman acts, comprising forced transfer, enforced disappearances and attacks against human dignity".
Between 1.7 million and 2.2 million people are believed to have died under the Khmer Rouge. Led by the late Pol Pot, the regime sought to turn Cambodia back to "year zero" in its quest for a peasant utopia.
During the trial that started in November 2011, Nuon Chea admitted being "morally responsible" for the bloodshed, while Khieu Samphan expressed regret but said he was only a figurehead in Pol Pot's regime.
There were initially four defendants, but former Foreign Minister Ieng Sary died in 2012 and his wife and ex-minister Ieng Thirith, who was known as the "First Lady of the Khmer Rouge", has Alzheimer's disease and was ruled unfit for trial.
Analysts have said Thursday's verdict might be the last one for the court, given the age and poor health of the defendants. Nuon Chea took part in much of the proceedings via video link.
The tribunal's first verdict was a life sentence for Kaing Guek Eav, or Duch, who ran the notorious Tuol Sleng prison, a converted school where as many as 14,000 people were tortured and executed. In his testimony, he insisted he was following orders and would have been killed himself if he had disobeyed.
The court had spent over $200 million by last year but has been plagued by disputes, resignations, funding shortages and accusations of political interference. Further cases are being investigated, but no new indictments have been made.
"The victims have seen the perpetrators brought to account before a court of law," said James Goldston, executive director of the Open Society Justice Initiative, which has monitored the tribunal.
"They have been tried fairly and found guilty. The historic significance of today's judgments should in no way be diminished by the very real challenges that have at times beset this process."
(Writing by Martin Petty; Editing by Alan Raybould)