A UN-backed war crimes tribunal in Cambodia has charged two former cadres of the 1970s Khmer Rouge regime with crimes against humanity over their alleged roles in the deaths of an estimated 1.8 million people.

The charges were brought against Im Chaem, a Buddhist nun in her 60s suspected of running a forced labour camp, and Meas Muth, a former navy chief in his 80s who allegedly sent detainees to a torture centre where at least 14,000 people died.

Neither suspect has yet to be arrested.

The announcement marked a rare sign of progress by a court plagued by infighting, political interference, resignations and funding shortages since it was set up nearly a decade ago to bring to justice "those most responsible" for the deaths of a fifth of the population from 1975-1979.

It has so far delivered guilty verdicts for crimes against humanity to only three defendants, two of whom - "Brother Number Two" Nuon Chea and former President Khieu Samphan - remain on trial in a separate case. One defendant died before a ruling was made and another was declared unfit for trial with Alzheimer's disease.

The latest charges cover breaches of Cambodian and international laws and conventions, including murder, extermination, slavery and political and ethnic persecution as well as "other inhumane acts".

Victims of the Killing Fields era perished from torture, disease, execution, starvation and exhaustion in the late Pol Pot's pursuit of a peasant utopia. Virtually every Cambodian alive today lost relatives.