A former female minister in Cambodia's Khmer Rouge government has lost an appeal against her continued detention.

Ieng Thirith, who held the post of social affairs minister, is being held by a United Nations-backed tribunal on charges of crimes against humanity.

The chair of the five judge panel said that keeping her in detention ahead of her formal trial remained "a necessary measure".

Prak Kimsan said there were "well-founded reasons to believe that crimes being investigated against the charged person were committed as part of systematic attack" during the Khmer Rouge regime.

The tribunal is holding five former senior Khmer Rouge officials pending trials expected to begin later this year related to atrocities committed during its rule. 

Among them is Ieng Thirith's husband, Ieng Sary, who was the foreign minister in the Khmer Rouge government.

He lost his own appeal against his continued detention by the trubunal last week.

Some estimates say up to 2 million people died during the Khmer Rouge's four years in power in the late 1970s.