Cambodia's UN-backed war crimes court has for the first time issued genocide charges against two leaders of the brutal Khmer Rouge regime, a tribunal spokesman said Wednesday.

Former Khmer Rouge number two Nuon Chea and foreign minister Ieng Sary were both charged over the hardline communist regime's slaughter of Vietnamese people and ethnic Cham muslims during the 1970s, spokesman Lars Olsen told AFP.

"This week both Nuon Chea and Ieng Sary have been brought before the investigating judges and informed they are being charged with genocide against the Cham muslims and the Vietnamese," Olsen said.

"This is the first time that anyone has been charged with genocide" at the UN-backed tribunal, he added.

The court announced last month it was investigating incursions into Vietnam as well as executions of Cambodia's Cham minority committed by the 1975-1979 regime.

Final arguments were heard last month in the trial of prison chief Kaing Guek Eav, known by the alias Duch, who was charged with war crimes, crimes against humanity, torture and premeditated murder in the court's first trial.

Both Nuon Chea and Ieng Sary have already been charged with war crimes and crimes against humanity.

They are in detention at the court, awaiting trial in the tribunal's second case along with Ieng Sary's wife, former social affairs minister Ieng Thirith and former head of state Khieu Samphan.

Led by Pol Pot, who died in 1998, the Khmer Rouge emptied Cambodia's cities in a bid to forge a communist utopia, wiping out up to two million people through starvation, overwork, torture and execution.

There are now nearly 240,000 Cham Muslims in Cambodia, mainly in the central provinces, forming 1.6 percent of the population in the predominantly Buddhist country, according to a recent survey by the US-based Pew Research Centre.