A Khmer Rouge jailer who oversaw the deaths of some 15,000 people was given life in prison on appeal Friday, bringing down the curtain on a landmark first case at Cambodia's UN-backed war crimes court.

Survivors of the regime's reign of terror hailed the decision to increase the original jail term of 30 years handed in 2010 to Kaing Guek Eav, better known as Duch, for war crimes and crimes against humanity.

Judges said the initial punishment given to the former head of the S-21 prison did not "reflect the gravity of the crimes" committed in the late 1970s.

"The crimes by Kaing Guek Eav were undoubtedly among the worst in recorded human history. They deserve the highest penalty available," said Kong Srim, president of the court's highest appeal body.

Wearing a white shirt and a beige jacket, the 69-year-old former maths teacher sat impassively in the dock as the verdict was read out, a brief pursing of the lips the only sign of emotion.

He had appealed for an acquittal on grounds that he was just following orders, but prosecutors also appealed saying the original sentence was not long enough.

The verdict pleased the hundreds of Cambodians, including orange-robed monks and elderly survivors of the brutal 1975-1979 regime, who packed the Phnom Penh court's public gallery to witness the conclusion of Duch's case.

"I'm happy now," said farmer Kim Huoy, who lost 19 family members, including her husband and parents, under the Khmer Rouge.

"It is good that there is justice for victims finally. I will feel peace in my mind now."

Led by Pol Pot, who died in 1998, the Khmer Rouge was responsible for one of the worst horrors of the 20th century, wiping out up to two million people through starvation, overwork and execution.

S-21, also known as Tuol Sleng prison, was the centre of the Khmer Rouge security apparatus and thousands of inmates were taken from there for execution in a nearby orchard that served as a 'Killing Field'.

Duch was the first Khmer Rouge cadre to face the international tribunal. His trial attracted huge interest in a nation still haunted by the brutality of the regime. The milestone final judgement was broadcast live on television.

The original verdict had outraged victims because it meant Duch could have walked free in under 18 years given time already served.

Before being taken back to the detention facility where he will spend the rest of his days, Duch briefly pressed his hands together in a traditional greeting to the judges.

During his nine-month trial Duch repeatedly apologised for his role at S-21, but surprised the court by asking to be acquitted at the 11th hour.

On appeal, Duch told judges he only survived because he 'respectfully and strictly followed the orders' but prosecutors argued that the shock request for a full acquittal showed Duch lacked remorse.

The final judgement will likely be hailed as a victory by the tribunal, regularly criticised for proceeding too slowly and dogged by allegations of political interference.

Amid the controversies, donor funds to the court have dried up in recent months and hundreds of Cambodian employees are currently not receiving their salaries.

A second trial involving the regime's three most senior surviving leaders opened late last year, but there are fears that not all of the defendants, who are in their eighties, will live to see a verdict.