Cambodia's war crimes court has ordered the release of Ieng Thirith, dubbed the "First Lady" of the murderous Khmer Rouge regime, saying she was unfit to stand trial.

"As there is no prospect that the accused can be tried in the foreseeable future, the trial chamber has confirmed the severance of the charges" against the 80-year-old, a statement from the UN-backed tribunal, released on Thursday, said.

Explaining its decision to stay proceedings against Ieng Thirith -- who was the sister-in-law of regime leader Pol Pot -- the court said her "cognitive impairment is likely irreversible".

One of only a handful of people ever brought before a court over the 1975-1979 regime, blamed for the deaths of up to two million people, Ieng Thirith was accused of war crimes, genocide and crimes against humanity.

The tribunal said the order to release her "is not a finding on the guilt or innocence of the accused, nor does it have the effect of withdrawing the charges against the accused".

Three other ageing top former regime leaders -- including her husband, former foreign minister Ieng Sary -- remain on trial.