Cambodia's U.N.-backed war crimes tribunal has charged the Khmer Rouge's former head of state with genocide, the third such charge this week against a former leader of the brutal regime.

Khieu Samphan is the most senior Khmer Rouge leader to face genocide charges in addition to other charges related to war crimes and crimes against humanity following the deaths of 1.7 million people during the 1975 to 1979 bloody reign by the Khmer Rouge.

Similar charges of genocide were issued earlier this week against Nuon Chea, the group's top ideologist, and former foreign minister Ieng Sary.

All three have been charged with involvement in the deaths of members of the country's ethnic Cham and Vietnamese communities.

The three are expected to face trial late next year once the prosecution and defense teams complete their investigations.

Lars Olsen is a spokesman for the tribunal. He says genocide charges are complicated. "It is not enough to actually prove that atrocities have been committed," he said. "You also need to prove there was intent behind the atrocities to destroy, in whole or in part, a group."

He says the intent of destruction is based on a group's ethnicity, nationality, religion or race and the current genocide charges relate directly to the alleged slaughter of ethnic Vietnamese and Muslim chams by the Khmer Rouge.

Seventy-eight-year-old Khieu Samphan is a French-educated former guerrilla leader who was arrested in 2007 and has since portrayed himself as a virtual prisoner of Pol Pot's regime. A self-styled intellectual, he has denied any knowledge of the atrocities.

The additional charges came three weeks after the end of the first trial of a senior Khmer Rouge leader. Kaing Guek Eav, better known as Duch, stands accused of overseeing the torture and extermination of at least 14,000 people at the infamous S-21death camp.

A verdict in that case is expected by late March or early April.