The only female Khmer Rouge leader on trial for genocide at Cambodia's UN-backed genocide tribunal is suffering from dementia and memory loss, a health expert said on Monday.

The medical assessment added to speculation that former social affairs minister Ieng Thirith, 79, could be deemed unfit to answer charges over the deaths of up to two million people during the regime's 1975-79 rule of terror.

A court-appointed geriatrics expert told judges at the Extraordinary Chambers in the Courts of Cambodia that Ieng Thirith, the sister-in-law of Khmer Rouge leader Pol Pot, had memory problems and needed further psychiatric assessment.

"Dementia has a number of causes. The most common is Alzheimer's," said professor John Campbell, referring to the progressive brain disease that causes memory loss and behavioral changes.

Ieng Thirith's trial alongside three other senior Khmer Rouge leaders on charges including genocide, war crimes and crimes against humanity officially opened in June but has been held up by health issues surrounding the defendants.

Observers say the process to determine Ieng Thirith's fitness could take months, likely delaying the trial until next year.

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

In July, her lawyers said they were unable to take instructions from her, citing mental problems.

"Brother Number Two" Nuon Chea, seen as the brutal regime's chief ideologue, is also contesting his ability to stand trial, although Campbell said he found no major concerns.

In a brief address to the court, during which he removed his trademark sunglasses, the 85-year-old accused said he had trouble concentrating and sitting for long periods.

He said Campbell had "failed to assess this issue concerning my concentration" and asked to be examined by another specialist.

Co-accused Ieng Sary, the regime's foreign affairs minister and ex-head of state Khieu Samphan are not contesting their ability to stand trial.

In its historic first trial, the court sentenced former prison chief Kaing Guek Eav - better known 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 overwork, starvation or execution in a bid to create an agrarian utopia.

(AFP)