Jailed Khmer Rouge leader Nuon Chea will remain in tribunal detention ahead of his atrocity crimes trials, a panel of judges ruled Thursday.

Nuon Chea's lawyers had argued his health had worsened since he was jailed in September, but the panel, led by Judge Prak Kimsan, chief of the Pre-Trial Chamber, ruled he could flee, damage evidence or intimidate witnesses.

"There is enough evidence for the provisional detention," Prak Kimsan said.

Nuon Chea, 82, is the senior-most of five jailed Khmer Rouge leaders in tribunal custody.

Son Arun, his lawyer, told journalists he "regretted" the decision and that Nuon Chea's mental health had deteriorated since his arrest.

"If the court wants to try him to the end, they must take care of Nuon Chea's health," Son Arun said. 

The decision was fair and up to international and national standards, said Hisham Mousar, a tribunal monitor for the rights group Adhoc. But he warned that the case of Ieng Sary may be different, because his health problems are more urgent.

Prak Kimsan also ruled Thursday that civil parties would be allowed to participate in future hearings, as important parts of initial investigations and national reconciliation.

The tribunal, meanwhile, announced former social affairs minister Ieng Thirith's pre-trial hearing was scheduled for April 21, and former nominal head Khieu Samphan's April 23.