Cambodia's former president under the Khmer Rouge, Khiew Samphan, has been charged with genocide by the UN-backed war crimes court.

He is the most senior Khmer Rouge leader to face charges in connection with the deaths of 1.7m people during the 1975-79 ‘Killing Fields’ reign of terror.

Similar charges of genocide were issued on Wednesday against ‘Brother Number Two’ Nuon Chea and former Foreign Minister Ieng Sary for their alleged roles in the slaughter of Cambodia's ethnic Vietnamese and Cham Muslim minorities during the Khmer Rouge regime.

All three have already been charged with war crimes and crimes against humanity, along with two other former leaders of the Khmer Rouge, who pursued a bloody agrarian revolution from 1975 to 1979 under the leadership of Pol Pot, who died in 1998.

It comes three weeks after the end of the first trial of a senior Khmer Rouge cadre.

Kaing Guek Eav, better known as Duch, was accused of overseeing the torture and murder of more than 14,000 people.

A verdict in that case is expected by March.

Khieu Samphan, aged 78, a French-educated guerrilla leader, was arrested in 2007.

He has portrayed himself as a virtual prisoner of the regime and denied knowledge of any atrocities.