Phnom Penh - The first survivor of a notorious Khmer Rouge torture prison to appear before Cambodia's UN-backed war crimes tribunal broke into tears Monday as he recounted the brutal techniques used to extract confessions from his fellow inmates.

Vann Nath, 63, who is one of a handful of survivors from S-21 prison in Phnom Penh, wept as he recalled conditions at the facility where at least 15,000 men, women and children were tortured before being sent to be murdered at the Cheong Ek "killing field."

"What happened there, these memories cannot be erased," he said. "I have tried to forget what happened but these memories haunt me."

Vann Nath's testimony came in the trial of former S-21 warden Kaing Guek Eav, known by his revolutionary name Duch, who is facing charges of crimes against humanity, premeditated murder and breeches of the Geneva Conventions, allegedly committed during the Khmer Rouge's 1975-1979 rule.

Duch, 66, has admitted guilt for his crimes and faces a maximum sentence of life in prison.

Vann Nath, whose famous paintings depict the horrors of the prison, told the court he was allowed to live because he was instructed to paint portraits of Khmer Rouge leader Pol Pol and other senior cadres.

"If I made the portrait attractive then I knew that I would be spared execution," he said.

A selection of Vann Nath's paintings presented to the court depicted dozens of prisoners shackled together in group cells, inmates being burned and tortured with sharp objects, and infants being torn from their mothers' arms.

Up to 2 million people died through execution, starvation or overwork during the Khmer Rouge's campaign to transform Cambodian society into an agrarian socialist utopia.

Duch is one of five former leaders facing trial before the tribunal, which was established in 2008 after a decade of negotiations between the Cambodian government and the UN. (dpa)