But the head of the prison — the first senior Khmer Rouge figure to face trial in the U.N.-assisted tribunal — denied it.
"It's hard for me to believe that the prisoner was burned alive. I believe that nobody would dare to violate my order," Kaing Guek Eav, better known as Duch, told the court. "They had to be killed and then burned to ash."
Up to 16,000 people were tortured under Duch's command at S-21 prison and later were taken away to be killed during the Khmer Rouge's 1975-1979 rule of Cambodia. Only a handful survived.
Cheam Soeu, now 52, told the court he was a youth when he joined the Khmer Rouge and helped the communist regime take power. He spent two years as a guard at S-21, where four Westerners were among the prisoners.
The Westerners included an American, an Australian, a New Zealander and a Briton, who were captured on their yacht while sailing in Cambodian waters.
Cheam Soeu told the court he was on guard outside the prison late one evening and watched as one of the Westerners — he does not know which — was led by three security guards to the street.
"The prisoner was still alive. They asked him to sit down, and they put a car tire over his body," Cheam Soeu testified. He said guards then set the body on fire. "I saw the charred torso of the body and black burned legs."
Cheam Soeu said he constantly feared that if he did something wrong he would face the same fate.
Some 1.7 million Cambodians died of torture, summary executions, disease and starvation during the Khmer Rouge's rule, during which the Maoist ideologues emptied cities and forced virtually the entire population to work on farm collectives.
Duch (pronounced DOIK) testified previously that he carried out orders from the regime's late leader Pol Pot to kill the four Westerners and then burn their corpses. Prison records suggest there may have been as many as 11 Western prisoners.
Duch, 66, is the only senior Khmer Rouge figure to acknowledge responsibility for his actions. He is charged with crimes against humanity and is the first of five defendants scheduled for long-delayed trials.