PHNOM PENH - Prosecutors demanded a 40-year jail term Wednesday for Khmer Rouge prison chief Duch, as he expressed "excruciating remorse" for the deaths of 15,000 Cambodians at his brutal torture centre.

The country's UN-backed war crimes court heard closing arguments from both sides in the tribunal's first trial delving into the horrors of the communist regime behind the "Killing Fields" atrocities three decades ago.

Under their leader Pol Pot, the Khmer Rouge wiped out nearly two million people through starvation, overwork and execution in their bid to turn Cambodia back to a rural "Year Zero" between 1975 and 1979.

International prosecutors said appeals for forgiveness by Duch -- whose real name is Kaing Guek Eav -- did not amount to a full guilty plea for his time in charge of the notorious jail known as S-21, or Tuol Sleng.

"We submit... that the sentence to be submitted by this trial chamber should be 40 years in prison," prosecutor Bill Smith told judges.

"In imposing this penalty, you are not taking away the accused's humanity but you are giving it back to the victims of S-21," he said.

"Let's recall that unlike the prisoners at S-21, he is being met with open and even-handed justice."

Duch's crimes warranted the maximum life sentence, but previous periods of detention, plus his partial acceptance of responsibility and cooperation with prosecutors, meant that they were seeking only a 40-year term, Smith added.

The 67-year-old former maths teacher faces charges of crimes against humanity, war crimes, torture and premeditated murder. A verdict is not expected until early next year.

Duch later made his concluding remarks from the dock, accepting responsibility for the estimated 15,000 men, women and children who died at the jail.

But he said anew that he was obeying orders to protect himself and his family.

"I found I ended up serving a criminal organisation which destroys its own people in an outrageous fashion. I could not withdraw from it, I was just like a cog in a ... machine," Duch said.

"To the survivors I stand by my acknowledgement to all crimes. As for the families of victims, my wish is that you kindly leave your door open for me to make my apologies," he said.

"In order to express my most excruciating remorse, I have fully and sincerely cooperated with the court whenever it is needed of me."

Defence lawyer Kar Savuth argued that Duch was a scapegoat.

"Under the Khmer Rouge there were 196 regime prison chiefs, so Duch alone should not be charged if these others enjoy immunity," he said.

One of the handful of people to survive Tuol Sleng questioned Duch's sincerity.

"For me, I want him to be hanged or jailed for life," said Chum Mey, who only lived through his time at the jail because his skills as a mechanic proved useful to the Khmer Rouge.

The trial has heard how inmates at Tuol Sleng -- a former high school -- had toenails and fingernails pulled out, were waterboarded and had the blood drained from their bodies in primitive medical experiments.

Their agonies detailed by Duch's meticulous record-keeping, inmates were forced to give false confessions of betraying the regime or working for foreign intelligence services.

Most prisoners were taken to a so-called "Killing Field", an orchard at Choeung Ek, near Phnom Penh, where they were bludgeoned to death with steel clubs.

The prison and the orchard now form a genocide museum.

The Khmer Rouge were toppled by Vietnamese-backed forces in 1979. Pol Pot died in 1998.

For Cambodians the controversial tribunal, established in 2006 after nearly a decade of negotiations between Cambodia and the United Nations, is the last chance to find justice for the Khmer Rouge's crimes.

Duch has been detained since 1999, when he was found working as a Christian aid worker in the jungle.

The joint trial of four other more senior Khmer Rouge leaders is expected to start in 2011.