Lawyers for a top Khmer Rouge leader on trial for atrocities said Tuesday they were considering quitting Cambodia's UN-backed court because government interference had tainted the proceedings.

The defence team for "Brother Number Two" Nuon Chea said a guilty verdict against their client was a foregone conclusion because the trial was marred by "a lack of experienced judges and a lack of independence".

"We are seriously wondering why we're here," Michiel Pestman told AFP, referring to himself and two other international co-lawyers defending Nuon Chea, the deputy leader of the brutal 1975-1979 regime.

"The outcome will be exactly what the government wants. If (prime minister) Hun Sen says my client is a genocidal killer, Cambodian judges know what to do," the Dutchman said. "The question is how long I feel my presence will be justified."

Prime Minister Hun Sen, himself a Khmer Rouge member before he defected, was quoted telling Vietnamese media in January that Nuon Chea was "a killer and genocide (perpetrator)" whose testimony was "deceitful".

The Khmer Rouge tribunal consists of a mix of national and international legal officials, with Cambodian judges making up the majority. Pestman said none of the Cambodians would "dare to disagree" with the premier.

Nuon Chea and his co-accused, ex-foreign minister Ieng Sary and former head of state Khieu Samphan, deny charges including crimes against humanity and genocide for their roles in a regime blamed for the deaths of up to two million people.

Allegations of political meddling have long dogged the court, although they have usually been driven by the government's opposition to any more prosecutions after the current trial ends.

But Nuon's Chea counsel insist the case currently underway has also been influenced.

Pestman said he was frustrated by judges' attempts to silence his team whenever they mentioned the role of senior government figures under the Khmer Rouge.

"Every time we raise an issue that is unpopular or threatens to expose interference, they turn the microphone off," he said.

Explaining why he had not yet quit, Pestman said: "I feel my client still appreciates my efforts even though he realises, like I do, that they are completely fruitless."

Led by "Brother Number One" Pol Pot, who died in 1998, the Khmer Rouge wiped out nearly a quarter of the population through starvation, overwork or execution in a bid to create an agrarian utopia.

The court has so far completed just one trial, sentencing a former prison chief to life in jail for overseeing the deaths of some 15,000 people.