The judge in the genocide trial of former Khmer Rouge leaders has ordered another examination of one of the defendants to determine her fitness to stand trial.

Judge Nil Nonn said Ieng Thirith, the former social affairs minister in the brutal Khmer Rouge government in the 1970s, is being held in provisional detention until a new evaluation of her mental fitness is made, a report by Voice of America said.

Judges ruled last month before the trial began Thirith was suffering with what appeared to be Alzheimer's disease and the charges against her would be stayed pending examinations of her health.

But the trial continues with the three other former leaders, including her husband, standing charges.

Thirith, 79, is the wife of Ieng Sary, 86, the regime's minister of foreign affairs and on trial along with the Khmer Rouge's main ideologist, Nuon Chea.

Chea, 85, was "Brother Number Two" to "Brother Number One," Pol Pot, who died in 1998. Pot fled the country after an invading Vietnamese army toppled the regime in 1979.

Khieu Samphan, 80, who served as president, makes up the group of four in one of Cambodia's most controversial trials that has stirred memories of the short but deadly Khmer Rouge regime. Thousands of civilians died in what are known as the Killing Fields.

An estimated 1.7 million Cambodians -- one-fifth of the population -- lost their lives under the Maoist Khmer Rouge that lasted from 1975-79. Hundreds of thousands of Cambodians fled the country into exile and uncertain refugee status.

The Khmer Rouge abolished religion, schools and currency to create an agrarian utopia through forced migration from the cities into the countryside. But most are believed to have died of starvation, overwork or execution by government officials and the military.

Bringing former Khmer Rouge leaders to trial has been difficult. A joint tribunal was established in 2006 but has ruled in only one case so far.

A 67-year-old math teacher, Christian convert and Khmer Rouge cadre Kaing Guek Eav, was given a 35-year jail sentence for crimes against humanity in July 2010. But the sentence was reduced to 19 years for time spent already in jail.

Eav, commonly known as comrade Duch, was director of the notorious prison and feared interrogation center Tuol Sleng, or S-21, in the capital.

An estimated 16,000 men, women and children were systematically tortured, many beaten to death, at the prison. Cambodian officials said only 14 people survived Tuol Sleng. Duch was alleged to have ordered the executions of 160 children in a single day.

Despite his stated remorse, Duch always contended he was simply carrying out orders from his superiors and did so because he feared for his own life.

At the start of the trial of the four defendants in November, Chea said he had nothing to do with the deaths and denied involvement in torture, saying he was serving the nation to protect it from foreigners.

"My position in the revolution was to serve the interests of the nation and people," he said during his 90-minute speech in the court.

"I had to leave my family behind to liberate my motherland from colonialism and aggression and oppression by the thieves who wished to steal our land and wipe Cambodia off the face of the Earth."

Last month in court Chea blamed the Vietnamese army for the deaths.

But delays have hit the tribunal's attempts to start other trials of people suspected of crimes against humanity, not least because of perceived interference from the government, whose leader, Prime Minister Hun Sen, was a one-time mid-ranking Khmer Rouge commander.

This week the Swiss co-judge on the tribunal clashed in public with his Cambodian colleague.

Laurent Kasper-Ansermet said he was unable to provide updates on the progress of investigations because his Cambodian counterpart, You Bunleng, would not agree to it, the BBC reported.

Bunleng responded, saying Kasper-Ansermet was not yet legally accredited and lacked understanding of the legal principles of the tribunal's work.

Kasper-Ansermet arrived in Cambodia in early December to replace an outgoing German judge, Siegfried Blunk. Blunk and Bunleng were heavily criticized by Human Rights Watch for not being impartial.