The prosecution at Cambodia's war crimes tribunal called Wednesday for judges to hand down a 40-year jail term to Comrade Duch, the former commander of the Khmer Rouge's most notorious prison known as S-21.

Co-prosecutor William Smith said Duch, whose real name is Kaing Guek Eav, was the Khmer Rouge regime's 'loyal and dedicated agent' in running S-21, where thousands of people were tortured and executed between 1975 and 1979, and must be punished accordingly.

Despite some cooperation by Duch and his limited acceptance of his responsibility, Smith said the loss and suffering of the victims and their families must be foremost in the judges' minds when considering their sentence.

The prosecution accepted that their proposed 45-year tariff ought to be reduced by five years for time already served by Duch in pre-trial detention.

Duch is 67, so the prosecution's jail demand would effectively translate to life in prison.

'The whole of humanity demands a just and proportionate response to the crimes [committed at S-21],' he said, adding that the court must remember that the lives and dreams of thousands murdered at S-21 had been stolen by Duch.

Sentencing in Duch's trial is due to take place early next year. There is no death penalty in Cambodia, and he faces a maximum punishment of life in prison.

Duch, who appeared in court in a lilac Ralph Lauren shirt and beige trousers, ran the infamous torture and execution centre known as S-21 in Phnom Penh where at least 15,000 people died between 1975-79. Very few inmates survived.

On Tuesday, the national co-prosecutor Chea Leang told the court that far from being the mere functionary he had tried to portray himself as, Duch was in fact the key intelligence operative for the Khmer Rouge as they sought out real and imaginary enemies of the revolution.

'He was the [Khmer Rouge's] trusted man to identify supposed plots against the revolution and to root out enemies,' Chea Leang said.

Thousands of people were tortured and killed at S-21. Witnesses at the trial this year have told how some prisoners had their blood entirely drained, while others suffered simulated drowning, electrocution and beatings.

The defence and Duch himself are scheduled to address the court later Wednesday.

On Monday, lawyers for the civil parties, a collection of 90 people who are predominantly relatives of prisoners who were murdered at S-21, presented their closing arguments.

Duch has testified during the 72-day trial that he was simply following orders and had almost no power to help detainees sent to S-21.

Four senior surviving Khmer Rouge leaders are currently in jail and awaiting trial.

The Khmer Rouge ruled Cambodia between 1975 and 1979 before being forced out of power by neighbouring Vietnam. Around two million people died of starvation and disease or were executed under the radical regime.

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