PHNOM PENH - Cambodia's UN-backed court heard chilling details on Wednesday of the mass murder of Muslim women by Khmer Rouge soldiers, the latest grisly testimony in the trial of two top leaders of the ruthless communist regime.

"Brother Number Two" Nuon Chea, 89, and the former head of state Khieu Samphan, 84, are being tried for genocide of ethnic Vietnamese and Muslim minorities, as well as for their regime's use of forced marriage and rape.

The pair have already been handed life sentences in a previous trial that focused on the regime's forced evacuation of Phnom Penh into rural labour camps and murders at an execution site.

Up to two million Cambodians died under Khmer Rouge rule from 1975-1979, including an estimated 100,000 to 500,000 Cham Muslims and 20,000 Vietnamese.

Before these charges were filed, the mistreatment of the two minority groups was rarely discussed.

Cham survivor Math Sor, who was a teenager when the regime took power, told the court she was captured with about 30 other women in her north-eastern village by Khmer Rouge cadres and tied up in a house.

Twenty of the captives - those who said they had Cham parents or were not fully Khmer - were then taken to a pit several metres away, she told the UN-backed court.

"I heard the screams of 'please don't rape me'," recounted Math Sor, who lost eight relatives, including her parents and two pregnant sisters, during the Khmer Rouge's brief but brutal rule.

She then watched soldiers behead the women through "a crack in the wall" of the house where she and the others were held.

The soldiers also forced Cham Muslims in her village to eat pork, speak Khmer, and cut their hair, she told the court.