Former Khmer Rouge "first lady" Ieng Thirith has lost her appeal against her pre-trial detention and will remain jailed by Cambodia's UN-backed genocide court awaiting trial for crimes against humanity.

The 76-year-old Thirith served as the regime's social affairs minister and is one of five former leaders likely to face prosecution for atrocities committed during the Khmer Rouge's 1975-79 rule over Cambodia. She was arrested in November 2007 with her husband, former regime foreign minister Ieng Sary.

"It is necessary to keep her in detention," said judge Prak Kimsang, who presided over the July 9 ruling announcement at the Extraordinary Chambers. Judges added that Thirith was a flight risk and could put pressure on witnesses if released.

Lawyers for Thirith, who was the Khmer Rouge regime's most powerful woman, said the ruling was unfair, arguing that their client was too infirm to try and flee the country.

“My client does not accept the decision made by judges at the pre-trial chamber,” lawyer Phat

Pov Seang told reporters following the hearing.

"Based on medical examinations, she is elderly and always sick and could not escape to anywhere," he added.

Thirith, who had appeared before the court on May 21, has denied any wrongdoing under the Khmer Rouge, claiming that she only helped people and built hospitals during the regime.

She is the only female Khmer Rouge leader detained by the tribunal, which was convened in 2006 to find justice for victims of one of the 20th century's darkest chapters.

Some 1.7 million people died of starvation, disease and overwork, or were executed as the ultra-communist Khmer Rouge exiled Cambodia's population onto vast collective farms in its bid to forge an agrarian workers' utopia.

Among the other detainees are Khmer Rouge leader Pol Pot's top lieutenant, "Brother Number 2" Nuon Chea, former head of state Khieu Samphan and Kaing Guek Eav, the head of the regime's main prison Tuol Sleng who is better known by his revolutionary name Duch.

The first public trials are expected to start in September amid fears that some of the aging defendants could die before going to court. Pol Pot died in 1998, while another likely defendant, military commander Ta Mok, died in 2006.