A UN-backed tribunal has sentenced a senior member of the Khmer Rouge to 35 years in prison, three decades after the "Killing Fields" revolution tore Cambodia apart.

The verdict was short of the maximum 40 years sought by the prosecution and of the life behind bars demanded by many Cambodians who have struggled for decades to find closure for one of the darkest chapters of the 20th century.

Kaing Guek Eav, known as Duch, has been found guilty of murder, torture, rape, inhumane acts, crimes against humanity and other charges for running a prison that symbolised the horrors of the ultra-communist regime blamed for 1.7 million deaths in 1975-79.

The 67-year-old former schoolteacher, who admitted to overseeing the torture and killing of more than 14,000 people, will only serve 30 years because the court ruled he was held illegally by the Cambodian military from 1999 to 2007.

Duch betrayed no emotion as the verdict was read but some Cambodians wept loudly in the courtroom. "There is no jusice. I wanted life imprisonment for Duch," said Hong Sovath, 47, sobbing. Her father, a diplomat, was killed in the prison.

The court said at least 12,273 people were killed at Duch's Tuol Sleng prison, a converted high school also known as S-21 but acknowledged the number could be as high as 14,000.

"The chamber has decided there are significant mitigating factors that mandate a finite term imprisonment rather than life imprisonment," the tribunal's president said, citing Duch's expressions of remorse and cooperation with the court.

The cases of former President Khieu Samphan, "Brother Number Two" Nuon Chea, ex-Foreign Minister Ieng Sary and his wife, Ieng Thirith are highly complex and politicised. Many fear they may never go to trial, or they might die before seeing a courtroom.