Cambodia's UN-backed war crimes court said Wednesday it had started contempt of court proceedings against Voice of America Khmer for revealing confidential information about a new Khmer Rouge case.

The move comes after the US-funded news service posted an article and video on its website describing prosecution allegations of mass killings and other atrocities by three mid-level cadres during the regime's 1975-79 rule.

The service cited a document obtained by a source close to the court.

VOA Khmer "on 10 August 2011 quoted verbatim from a confidential document... and even showed that document on a video," said the two judges who are still investigating the claims in the court's fourth and final case.

The news service chief Chris Decherd refused to comment directly on the court action, but said: "VOA Khmer supports the work of the tribunal and helps inform the Cambodian public by reporting about the court's work."

He added that VOA's role was to serve Cambodian citizens "who deserve and are well-served by objective and quality news reporting about issues and topics that impact and affect their daily lives".

This marks the first time judges have followed through on warnings to launch contempt proceedings, following numerous leaks to the media.

Their terse statement however failed to clarify if action was being taken against the journalist, the editor or the producer of the piece, all of whom are understood to be in Washington, DC.

"They have no power to enforce contempt sanctions against a journalist who is not in Cambodia," said Anne Heindel, a legal advisor to the Documentation Centre of Cambodia, which researches Khmer Rouge atrocities.

She added that the tribunal's rules against interfering with the administration of justice were "troublingly vague" and it was not clear what sanctions could be imposed.

In its landmark first trial, the tribunal last year sentenced former prison chief Kaing Guek Eav, better known as Duch, to 30 years in jail for overseeing the deaths of 15,000 people. That case is now under appeal.

A second trial involving four of the regime's most senior surviving leaders has stalled as the court mulls whether all the elderly defendants are fit to stand trial.

A third case, thought to involve two ex-Khmer Rouge commanders, is still under consideration but like the fourth one, it is widely expected to be dismissed in the face of political opposition.

The hardline communist Khmer Rouge movement oversaw one of the worst horrors of the 20th century, wiping out up to two million people through starvation, overwork and execution in a bid to forge an agrarian utopia.

A major Khmer Rouge trial at Cambodia's UN-backed war crimes court may not hear testimony until next year, observers said Wednesday, amid fears over the mental health of one of the defendants.

The trial against four top leaders of the brutal 1975-79 regime, which began in June, has stalled after a recent decision by judges to order a psychiatric assessment on Ieng Thirith, 79.

Anne Heindel, a legal advisor to the Documentation Centre of Cambodia, which researches Khmer Rouge atrocities, believes the process to determine mental fitness "is likely to take until the end of the year".

That view was echoed by a court official, who told AFP it is "unlikely the trial will get going this year" but asked not to be named.

The elderly defendants face charges of genocide, war crimes and crimes against humanity over the deaths of up to two million people, in the court's second and long-awaited trial.

Questions have long been raised over the mental state of the regime's "First Lady" Ieng Thirith, who famously lost her cool during a 2009 court appearance, telling her accusers they would be "cursed to the seventh circle of hell".

Ieng Thirith's lawyers have recently said they are unable to take instructions from her, citing mental problems.

Tribunal spokesman Lars Olsen refused to speculate on a start date but said the court would "shortly" appoint mental health experts to examine the ex-social affairs minister, who is married to co-defendant Ieng Sary.

Both Ieng Thirith and fellow accused "Brother Number Two" Nuon Chea also plan to contest their physical fitness to stand trial at a hearing next week.

The prospect of more delays will upset some victims, who fear not all of the suspects, aged 79 to 85, will live to see a verdict.

"The accused and I are all very old now," said Bou Meng, 70, who was one of few people to survive a notorious Khmer Rouge torture prison.

"I urge the court to work on this case as soon as possible, so that justice will not die with the corpses."

In its historic first trial, the tribunal sentenced former prison chief Kaing Guek Eav -- also known as Duch -- to 30 years in jail last year for overseeing the deaths of 15,000 people. The case is now under appeal.

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 executions in a bid to create an agrarian utopia.