The UN-backed tribunal, which is trying the regime's former prison chief Duch, has been plagued by allegations of political interference by the government and claims that Cambodian staff had to pay kickbacks for jobs.

'I came here to work as a judge... The day I will not be able to work anymore, I will leave,' said investigating judge Marcel Lemonde at a press conference.

The UN has withheld funds to Cambodian operations at the court in absence of an agreement to stop the alleged corruption, however the global body's talks with senior government officials have ended without a deal.

Mr Lemonde and co-investigating judge You Bunleng said their office had so far interviewed more than 450 witnesses but their work on the court's second case against four senior leaders would not be completed before the end of the year.

'We are very aware of the necessity to complete the investigation as soon as possible. Our goal is to complete it by the end of 2009,' Mr Lemonde said. However he added 'this goal is a very ambitious one, given the complexity of the file'. Two investigators have also been in France since last week to interview witnesses, court officials said.

The long-awaited first trial has heard Duch, whose real name is Kaing Guek Eav, acknowledge responsibility and beg forgiveness for overseeing the torture and execution of more than 15,000 people at the notorious Tuol Sleng prison.

Testifying on Wednesday, American Khmer Rouge expert Craig Etcheson said that among the network of regime prisons around the country, Tuol Sleng was the only one that worked directly for the regime's leaders.

'S-21 was unique. It was... considered an organ of the government or the communist committee,' Mr Etcheson said.

The widespread purges during the 1975 to 1979 regime were probably due to senior leaders' paranoia and a result of indiscriminate methods of weeding out Khmer Rouge enemies, Mr Etcheson said. -- AFP