Image caption Ieng Thirith is accused of committing crimes against humanity under the Khmer Rouge regime

Cambodia's UN-backed genocide court has delayed the release of Khmer Rouge ex-minister Ieng Thirith after prosecutors lodged an appeal to limit her freedom.

The Maoist regime's former social affairs minister was due to be freed on Friday after being deemed unfit to stand trial because of health problems.

Prosecutors have requested the surrender of her passport and "weekly safety checks" as part of their appeal.

Three other Khmer Rouge chiefs are also on trial for crimes against humanity.

Ieng Thirith, 80, was initially scheduled to be released on Friday after court officials said she probably had Alzheimer's disease and there was no prospect she could be tried "in the foreseeable future".

"On the basis of the court-appointed medical experts' report and testimony, the Trial Chamber has reaffirmed its prior finding that the accused Ieng Thirith suffers from a progressive, degenerative illness [probably Alzheimer's disease] and that she remains unfit to stand trial," the court said in a statement on Thursday.

It added, however, that this was "not a finding on the guilt or innocence of the accused".

Flight risk

Prosecutors agreed with Ieng Thirith's release but said it should not be unconditional.

"Factors such as national reconciliation and stability, justice, public order and safety must also be given due regard," a spokesperson said on Friday.

In addition to handing over her passport and ID card, prosecutors have asked that Ieng Thirith make herself available for a "weekly safety check", not contact any witnesses and undergo a medical exam by court-appointed doctors every six months, the Associated Press reported.

The measures are aimed at preventing her from fleeing the country and interfering with the ongoing trials of three other senior Khmer Rouge leaders.

Who were the Khmer Rouge? Maoist regime that ruled Cambodia from 1975-1979

Led by Saloth Sar, better known as Pol Pot

Abolished religion, schools and currency in effort to create agrarian utopia

Up to two million people thought to have died of starvation, overwork or by execution

Defeated in Vietnamese invasion in 1979

Pol Pot fled and remained free until 1997 - he died a year later

Officials estimate up to two million people died during the Khmer Rouge's 1975-1979 rule.

Under supreme leader Pol Pot, the regime tried to build a Maoist peasant utopia, but descended into genocide as leaders sought to eliminate anyone who they deemed posed a threat.

In November 2011, three top Khmer Rouge leaders - Nuon Chea, Khieu Samphan and Ieng Sary - went on trial for crimes committed during the regime's rule. All three deny the charges.

Ieng Thirith, who was Ieng Sary's wife and the most powerful woman among the Khmer Rouge top ranks, has also denied any wrongdoing.

However, prosecutors said she knew that people were dying from starvation and disease on collective farms, but did nothing to stop the brutality.

Her sister was married to the movement's leader, Pol Pot. He died in the late 1990s.

Only one senior Khmer Rouge figure has been convicted in connection with crimes committed under the regime.

Prison chief Duch, born Kaing Guek Eav, was jailed in 2010 for his role in running a notorious prison where thousands of inmates were killed.