PROSECUTORS at a UN-backed court in Cambodia yesterday demanded an increased sentence of life imprisonment for a former Khmer Rouge prison chief convicted of crimes against humanity.

Kaing Guek Eav was sentenced in July to 35 years for overseeing the deaths of 15,000 people in the late 1970s, reduced to 30 years on the grounds that he had been detained illegally for years before the tribunal was established.

"We are asking for life imprisonment," said international co-prosecutor Andrew Cayley, after his team submitted their appeal document.

But he said the sentence should be shortened to 45 years because of the unlawful detention.

Prosecutors said the request for a tougher sentence reflected doubts over the remorse of the former jailer, better known as Duch, who apologised during his trial but shocked the court in November by finally asking to be acquitted.

"Frankly now, after he said he should be released... we are not convinced about this man's contrition," said Cayley.

Duch, 67, was the first former Khmer Rouge cadre to face an international tribunal, and the ruling was hailed as a historic day for Cambodia.

But many victims of the brutal 1975-1979 regime were dismayed by the sentence, which also took into account time already served since Duch was first detained in 1999, meaning he could walk free in about 19 years.

In their appeal document, the co-prosecutors argued that "a sentence of 35 years for crimes of this magnitude is plainly unjust."

They are also seeking additional convictions "for the crimes against humanity of extermination, enslavement, imprisonment, torture, rape, persecution on political grounds, and other inhumane acts."

Duch's lawyers are also appealing the sentence, which they have described as a "miscarriage of justice," and want the jailer to be acquitted.

Led by "Brother Number One" Pol Pot, the Khmer Rouge was responsible for one of the worst horrors of the 20th century, wiping out nearly a quarter of Cambodia's population through starvation, overwork and executions.

Tuol Sleng prison, run by Duch, was at the centre of the Khmer Rouge security apparatus and thousands of inmates were taken from there for execution in a nearby orchard.