PHNOM PENH (AFP) - A rare survivor of the Khmer Rouge regime's main jail has told how torturers ripped out his toenails and gave him electric shocks to try to make him confess to being a CIA agent.

Former mechanic Chum Mey told Cambodia's UN-backed war crimes tribunal on Tuesday how he pleaded for his life as he was tortured for 12 days and nights at the 1975-1979 communist movement's Tuol Sleng detention centre.

The 63-year-old is the second survivor to give evidence at the trial of prison chief Duch, who is accused of overseeing the torture and extermination of 15,000 people who passed through the facility.

Chum Mey said he had been working at a sewing machine factory in 1978 when he was brought to Tuol Sleng to be tortured on suspicion of espionage.

"While I was walking inside I said (to a guard), 'Brother, please look after my family.' Then the person kicked me on to the ground," Chum Mey said, adding the man swore at him and told him he would be "smashed".

Chum Mey told judges he was photographed, stripped, handcuffed and yanked by his earlobes to interrogators.

"They asked me to tell them the truth -- how many of us joined the KGB and CIA," Chum Mey said, referring to the Soviet and US intelligence agencies. "I told them I did not know any CIA or KGB. Truly, I did not know those terms."

He went on to describe how interrogators beat him as he pleaded for his life, and proceeded to torture him for 12 days and nights.

He trembled in pain after they removed his toenails and heard "some sort of sound" after they electrocuted him, he said.

"The method used was always hot. It was never cold, as Duch has said," Chum Mey said, describing degrees of torture.

Earlier in his trial for war crimes and crimes against humanity, the 66-year-old Duch begged forgiveness from the victims after accepting responsibility for his role in governing the jail.

But he has consistently rejected claims by prosecutors that he had a central role in the Khmer Rouge's iron-fisted rule and says he never personally executed anyone.

Khmer Rouge leader Pol Pot died in 1998, and many believe the tribunal is the last chance to find justice for victims of the communist regime, which killed up to two million people.