Pol Pot's chief ideologue has testified in defence of a former Khmer Rouge commander on trial in Cambodia for his alleged role in the killing of three Western backpackers.

Nuon Chea - once known as "Brother Number Two" and the most senior surviving Khmer Rouge leader - told a court on Friday that the defendant had been transferred out of the region where the killings took place shortly before they occurred.

Nuon Chea supported Sam Bith's defence

Sam Bith, 69, is the most senior of three Khmer Rouge commanders charged with abducting the tourists after an attack on a train in southern Cambodia in 1994.

The backpackers, a Briton, a Frenchman and an Australian, were later executed and buried in the jungle, several weeks after ransom negotiations failed.

Rare appearance

The 77-year-old Nuon Chea - who rarely appears in public and was not expected to testify - said Sam Bith had complained of health problems in the spring of 1994.

In June 1994 - a month before the attack on the train - Pol Pot transferred him, Nuon Chea testified.

Nuon Chea also denied having anything to do with military matters during the Khmer Rouge's 1975-1979 rule.

Sam Bith says he was not in the area

"I was in charge of political education," he said. "Pol Pot was the one who appointed commanders, not me."

Sam Bith's denial that he was in the region where the kidnappings took place is a key part of his defence.

"Sam Bith was not involved in any of the attacks. The proof is that Pol Pot and Nuon Chea had ordered his transfer from the south-western zone in June 1994," before the ambush in July, his lawyer Ka Savuth said.

Sam Bith, who was arrested in May after eight years on the run, is charged with kidnapping, conspiring in premeditated murder, terrorism and robbery.

If convicted he could be sentenced to life in jail.

Train ambush

Australian David Wilson, Briton Mark Slater and Frenchman Jean-Michel Braquet, all aged in their late 20s, were abducted on 26 July, 1994.

At least 10 Cambodians died in the attack on the train.

Khmer Rouge General Nuon Paet was jailed for life in June 1999 for his role in the case. The Supreme Court later turned down an appeal against his conviction.

Another officer, Colonel Chhouk Rin, was sentenced in absentia to life imprisonment in September, but has vowed to appeal against the sentence.

During his trial, Nuon Paet said Sam Bith, who was his superior officer, had ordered the tourists killed.