Cambodia's United Nations-backed war crimes tribunal has ordered the release of Ieng Thirith, the aging sister-in-law of former Khmer Rouge leader Pol Pot.

On Thursday, the court said the 80-year-old – once dubbed the “First Lady” of the Khmer Rouge – is unfit to stand trial for genocide because of a degenerative illness, likely Alzheimer's disease.

The ruling, which upholds an earlier decision, found that all treatment options have been exhausted and that Thirith's sickness is “likely irreversible.”

She was charged with crimes against humanity, genocide, homicide, torture and religious persecution related to the 1975-1979 rule of the Khmer Rouge, which resulted in the deaths of some 1.7 million Cambodians.

The court statement said all charges against the former social affairs minister for the radical Communist movement have been dropped. But it said her release does not reflect a ruling on her guilt or innocence.

The tribunal is seeking justice for the legions of victims who died of starvation, execution or lack of medical care during the Khmer Rouge's reign.

Ieng Thirith and her three co-defendants are the most senior survivors of the regime's leadership. All defendants deny the charges. Pol Pot died in 1998.