A UN backed tribunal began in Cambodia on Monday trying the two surviving leaders of Khmer Rouge regime of the crime of genocide.

Last year both Khieu Samphan and Nuon Chea were found guilty of war crimes and crimes against humanity, but Monday’s trial will see them face charges of genocide. The trial will examine crimes carried out against ethnic Cham Muslims and ethnic Vietnamese during the Khmer Rouge’s 1975-79 rule.

The prosecution stated that 90 percent of the 200,000 ethnic Vietnamese population in the country were forcibly deported by the regime, and the remaining 20,000 were killed in a process that "involved mass killings of Vietnamese civilians who were sought out solely on the basis of their ethnicity."

The prosecution also alleged the Khmer Rouge "succeeded in physically destroying a significant portion of the Cham population, solely because of their ethnic and religious background."

This effort included removing Cham women and children from the group and placing them in Khmer communities, forcing Cham to marry outside their ethnic group, and banning all cultural aspects traditionally identified with that group," it said, adding the campaign "culminated in 1977 and 1978 with organized mass executions of entire Cham communities conducted by [Khmer Rouge] cadres in multiple [areas of Cambodia]".

Lars Olsen, the tribunal’s legal officer, said the case was limited to the two ethnic groups “because the legal definition of genocide is different from what many people would regard as genocide.”

“Clearly in this context of Cambodia where the majority of the Cambodians who were killed during the Khmer Rouge regime were killed by Cambodians with the same ethnicity, simply the traditional legal definition of genocide would not include these kinds of acts," he added.

See more from Voice of America here and Deutsche Welle here.