PHNOM PENH – A leader of the Cambodian Khmer Rouge expressed remorse on Thursday 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.

"I am very regretful for events that happened intentionally and unintentionally. I am morally responsible," he said, expressing "condolences" to victims of the regime present in the court, where he faces charges including war crimes and crimes against humanity.

"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 or even knowledge the killings.

Khieu Samphan said he regretted the "unspeakable suffering" done to the Cambodian people under the Khmer Rouge and offered condolences, his first such appology in court.

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

"Many victims have waited more than 30 years to hear any statement of apology or regret from leadership figures in the Khmer Rouge," he said.

However, Khieu Samphan continued to insist he was simply a figurehead of the regime and knew nothing about its murderous side.

"Looking at it from outside, people might assume that I was the big leader. Really, I just had big status, I had no real power to arrest anyone. I have no knowledge of the people's living conditions," he said.

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

To date, it has delivered one verdict, a life sentence given to Kaing Guek Eav, better known as Duch, chief of the notorious Tuol Sleng prison, a converted Phnom Penh school where as many as 14,000 people may have been executed.

The current trial opened in June 2011 with four people in the dock – Nuon Chea, Khieu Samphan, former Foreign Minister Ieng Sary and his wife, Ieng Thirith, who was social affairs minister in the Khmer Rouge government.

Ieng Thirith, a sister-in-law of Khmer Rouge leader Pol Pot, who died in 1998, was declared unfit to stand trial last year because she was suffering from Alzheimer's disease. Ieng Sary died in March.