Phnom Penh - A man who worked as a guard at the main Khmer Rouge torture centre admitted at Cambodia's UN-backed war crimes trial Thursday that he executed a prisoner at a "killing field".

    Him Huy, 54, was giving evidence at the trial of his former prison chief Duch, who has admitted responsibility for overseeing the torture and execution of around 15,000 people held at Tuol Sleng prison, also known as S-21.

Live feed of Him Huy's testimony on Thursday. (Photo: Reuters)

    The former guard recounted how he killed the prisoner in 1977 at Choeung Ek killing field, under orders from either Duch or from the prison chief's now-deceased deputy, Hor.

    "I'm not really clear at that time whether it was Duch or Hor because it was almost dawn and it was a rush to finish," Him Huy told the court.

    "He asked whether I was absolute or not. I answered that I was absolute, because I was afraid... After I received instructions, I executed that prisoner," he said.

    Earlier in the day, Him Huy told judges that all prisoners at S-21 were interrogated and then executed.

    The witness said prisoners were informed they were going "to a new home" and then trucked -- sometimes in loads of up to 100 people -- to Choeung Ek killing field, a former orchard on the outskirts of the capital Phnom Penh.

    "My force would guard those prisoners and the executioners would get ready at the pit. Guards would post at the gates and each prisoner would be walked to the pit to be killed," Him Huy said.

    "When they were killed, first they were asked to sit at the edge of the pit, then they would be struck, then their throats would be slashed, then (guards) would take off their clothes and their handcuffs," he added.

    Him Huy told the court that although he never saw Duch abuse a prisoner, the prison chief witnessed killings.

    "I saw him (Duch) twice at Choeung Ek. He stayed there until all detainees were executed and then he would leave," Him Huy said.

    He described Duch as someone who "spoke very softly" but was "very strict on his work performance". The witness said he grew increasingly afraid of the prison chief as he arrested staff at Tuol Sleng.

    "There were about 300 guards, but later on they were arrested continuously and at the end there were only about 50 or 60," Him Huy said.

    "The only thing I knew for sure is that people kept disappearing. I don't know what happened to them when they were arrested."

    The 66-year-old Duch, whose real name is Kaing Guek Eav, has accepted responsibility for his role governing the jail and begged forgiveness near the start of his trial for war crimes and crimes against humanity.

    But he has consistently rejected claims by prosecutors that he held a central leadership role in the Khmer Rouge, and maintains he never personally killed anyone.

    Led by Pol Pot, who died in 1998, the Khmer Rouge emptied Cambodia's cities in a bid to forge a communist utopia. Up to two million people died of starvation, overwork, torture and execution during the 1975-1979 regime.

    Four other former Khmer Rouge leaders are currently in detention and are expected to face trial next year at the court, which was formed in 2006 after nearly a decade of wrangling between the UN and the Cambodian government.

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