PHNOM PENH -- The former chief of the Khmer Rouge's notorious S21 torture center has been charged with crimes against humanity and will remain behind bars until a tribunal can be established, lawyers said Monday.

Kang Kek Ieu, also known as Duch, was scheduled for release on Thursday under special Cambodian laws but his lawyer, Ka Savuth, said an additional charge of crimes against humanity would ensure his client remains in prison.

Duch, now a born-again Christian, ran S21 during the 1975-79 reign of the ultra-Maoists, AFP reported.

Under his auspices more than 16,000 people were sent to their deaths from the holding camp -- established in a Phnom Penh high school -- where people were branded as intellectuals, a capital offence, for wearing glasses.

Rules included: "When receiving electrification you must not cry."

He was arrested on May 9, 1999 and charged with genocide.

But under Cambodian law Duch could only be detained for three years without facing trial. The additional charge will ensure he is not released within the short term.

"The charges were made not long after Ta Mok was charged, and Duch, like Ta Mok, was charged with crimes against humanity by the military courts," Ka Savuth said.

"I have submitted a request to have him released on bail but I have not received an answer," he said.

News of the charge was not released until Monday.

Ta Mok -- the Khmer Rouge former military chief -- was arrested on March 6, 1999 and charged with genocide. Three years later he was charged with crimes against humanity to prevent his release from jail under the three year release clause.

Efforts to hold an international tribunal capable of putting surviving Khmer Rouge leaders in the dock struck their biggest hurdle in February when the United Nations pulled out of negotiations citing Cambodia's inability to hold impartial hearings.

However, the Cambodian government has signalled it would proceed with a foreign-assisted trial without UN involvement or conduct a Cambodian-only tribunal.

Others cited for trial include "brother number two" Nuon Chea, former head of state Khieu Samphan, former foreign minister Ieng Sary and his wife, Ieng Tirith. All live freely in west Cambodia.

None of the Khmer Rouge leaders have faced trial for the deaths of about 1.7 million people through alleged genocide, forced labour, and starvation.

Senior figure Ke Pauk died in February heightening fears that surviving Khmer Rouge members would die of natural causes before facing justice.

Pol Pot, who was known as brother number one, died in his sleep in his jungle hideout in 1998, having never voiced a moment's regret for his disastrous experiment.