The Khmer Rouge tribunal's Supreme Court has ordered the regime's chief jailer to serve out the rest of his life in prison because of his "shocking and heinous" crimes against the Cambodian people.

The surprise ruling increased a lower court's 19-year sentence that had been appealed by prosecutors as too lenient and that had outraged survivors who feared the man who oversaw the killings of thousands could one day walk free.

Kaing Guek Eav, known as Duch, was commander of the top secret Tuol Sleng prison - code-named S-21. He admitted to overseeing the torture of his prisoners before sending them for execution at the "killing fields".

In July 2010, the tribunal's lower court convicted Duch (pronounced DOIK) of war crimes, crimes against humanity, torture and murder. He was sentenced to 35 years in prison but had 11 years shaved off for time served and other technicalities.

The sentence was appealed both by prosecutors who called for life imprisonment and by Duch who argued it was too harsh because he was merely following orders.

However, the judge said on Friday that the upper court felt the penalty should be more severe because the former jailer was responsible for the brutal deaths of so many. The tribunal says Duch oversaw the deaths of at least 12,272 victims but estimates have placed the number as high as 16,000.

"The crimes of Kaing Guek Eav were of a particularly shocking and heinous character based on the number of people who were proven to have been killed," the judge said.

The 69-year-old Duch stood calmly without emotion as the sentence was read. He then pressed his palms together and pulled them to his chest in a show of respect to the judge, before being led away by court guards. The ruling was final with no other chance for appeal.

The tribunal is seeking justice for an estimated 1.7 million people who died from torture, starvation, exhaustion or lack of medical care during the Khmer Rouge's 1970s rule.