With a curtain drawn to obscure his appearance and his likeness pixellated on court monitors as a protective measure, former killing fields guard Tay Teng testified at the Khmer Rouge tribunal yesterday on the atrocities he witnessed – and was at times party to – during the Democratic Kampuchea regime.

Teng, now in his late fifties, told the court about summary murders at the Choeung Ek killing fields on the outskirts of Phnom Penh, and described the disembodied screams he heard emanating from S-21, the regime’s infamous prison where he once stood guard.

Teng initially denied he had executed prisoners during his time stationed at Choeung Ek.

“I was not engaged in that kind of killing . . . My main tasks were to dig pits, bury the dead bodies and stand guard,” he said.

But when the prosecution confronted Teng with an interview he gave court investigators, he agreed he had personally executed prisoners. “In the first place, I did not know . . . Later on, I was so concerned after I learned about their plan. With the human heart, I had never done any such acts [before], but because of the duties that I had to perform, I had to do it.”

Teng described an eerie silence at the site, where he dug pits to fit more than 20 corpses each. Prisoners, he said, were told to sit at the rim of the pit, before they were “smashed” – often struck with a metal bar and finished with knives or palm-leaf blades.

Some prisoners, Teng testified, knew their fate as they were marched, handcuffed and crying, to the pits.

“They put their palms together to implore to spare their lives. Perhaps they knew,” Teng said.

He said that though three of his relatives were imprisoned at S-21, he was told to have an “absolute stance”, which meant he “should not have any emotional attachment to any of our relatives”.

Meanwhile, former S-21 photographer Nhem En finished his testimony before the court. After acknowledging under questioning that his main motivation for appearing was to garner publicity for his memoirs, En claimed to have the only photo “in the world” of purged former cadre Ruos Nhim.

Though it has been long sought-after by the trial chamber, En said he would have to “consider” the court’s request for a copy.