Survivors of the Khmer Rouge regime expressed scepticism Wednesday about an apology by the movement's brutal prison chief as his trial continued at a UN-backed court in Cambodia.

Duch told the tribunal on Tuesday that he felt "regret and heartfelt sorrow" and accepted responsibility for his role in the 1975-1979 communist regime that killed up to two million people.

"What is inside a human's heart is very secret — we cannot see it clearly," said Vann Nath, one of only about a dozen people who survived Duch's reign of terror at the notorious Tuol Sleng prison.

"We cannot totally believe that what he said is all true. We also cannot say that what he said is not true," added Vann Nath, who was only spared because the regime thought his artistic abilities were useful for propaganda.

Duch, 66, is being tried on charges of crimes against humanity, war crimes, premeditated murder and torture over the extermination of around 15 000 people at Tuol Sleng prison, also known as S-21.

On Tuesday he admitted blame for the crimes committed at the prison but said he was a "scapegoat" and had only been following orders from the Maoist movement's top leadership to protect his own family.

But many Killing Fields survivors said they believed that the former maths teacher was still concealing details about his role in the Khmer Rouge.

"He still hides some secrets. He said he did not commit the crimes by himself, but he has not yet pointed to (many) others," said comedian Prum Manh, who lost three brothers and his parents to the regime.

Buddhist monk Thuch Ngorn, who lost six members of his family, said he also wanted to hear more details about senior Khmer Rouge leaders.

"I forgive Duch, but many Cambodians won't forgive him because they lost their family members," the monk added.

The Irish photojournalist who found Duch in hiding a decade ago said, however, he thought it was important that the former torture chief was cooperating with the court.

"Everybody's asking 'Does he truly feel remorse?' But maybe that's the wrong question to ask," said Nic Dunlop outside the court.

Prosecutors on Tuesday recounted the prison's brutal torture techniques overseen by Duch and described him as a central figure in the Khmer Rouge's "widespread attack on the population of Cambodia."

Duch, who faces a possible life sentence, is one of five Khmer Rouge leaders in the custody of the court. The other four are due to go on trial later this year.

Many in Cambodia believe the UN-sponsored tribunal, formed in 2006 after nearly a decade of wrangling between the UN and the Cambodian government, is the last chance to bring regime figures still alive to justice.

But it has faced controversy over allegations of corruption and political interference.