Prosecutors for Cambodia's genocide tribunal on Monday appealed the 19-year prison sentence given to the Khmer Rouge's chief torturer, Kaing Guek Eav, also known as Comrade Duch.

Duch was found guilty of torture, murder, war crimes and crimes against humanity. He was originally sentenced in July to 35 years behind bars, but that term was reduced to 19 years after taking into account time served and other factors.

Lawyers for Cambodia's United Nations-backed Khmer Rouge genocide tribunal filed the appeal in the court on the outskirts of Phnom Penh on Monday, saying the judgment "gives insufficient weight to the gravity of Duch's crimes and his role and his willing participation in those crimes."

The reduced sentence meant that Duch, 67, could one day walk free.

Duch supervised the Tuol Sleng prison in Phnom Penh, where as many as 16,000 people were tortured and then executed in the late 1970s.

He was the first defendant before the tribunal, which is seeking justice for an estimated 1.7 million people who perished from execution, overwork, disease and malnutrition during the communist Khmer Rouge regime from 1975 to 1979.

Prosecutors said in their appeal that that mitigating circumstances were given too much weight. They also said "Duch should be separately convicted of the crimes against humanity of enslavement, imprisonment, torture, rape, extermination and other inhumane acts."

Four more defendants are expected to go on trial early next year:

• Nuon Chea, the Khmer Rouge ideologist.
• Khieu Samphan, its former head of state.
• Ieng Sary, its foreign minister.
• His wife, Ieng Thirith, who was minister for social affairs.