A leader of the Cambodian Khmer Rouge has expressed remorse for the deaths of an estimated 1.7 million people during the Killing Fields regime in the 1970s and accepted responsibility for the first time during court proceedings.

"I am responsible for what happened during the time of Democratic Kampuchea," Nuon Chea told the United Nations-backed tribunal, referring to the name of the country during the period, when he was the party's second-in-command.

"Brother Number Two" Nuon Chea and co-defendant Khieu Samphan, a former head of state during the Khmer Rouge period, have until now denied responsibility for the killings.

Samphan said he regretted the "unspeakable suffering" done to the Cambodian people under the Khmer Rouge and offered his condolences.

It was unclear why the two men had chosen to express remorse now, but Lars Olsen, a court spokesman, welcomed the admission.

The court, operated jointly by Cambodia and the United Nations, was set up in 2005 with the aim of trying those most responsible for the bloodshed.