PHNOM PENH - Cambodia's UN-backed court on Monday heard a former prisoner say he helped dig up more than 12,000 skulls in mass graves outside Phnom Penh, as the genocide trial of two Khmer Rouge leaders continued.
Nuon Chea, 88, known as "Brother Number Two", and former head of state Khieu Samphan, 83, face charges over the killing of ethnic Vietnamese and Muslim minorities, forced marriage and rape during the 1975-1979 regime.
In August the pair were given life sentences for crimes against humanity - the first top Khmer Rouge figures to be jailed - after a two-year trial focused on the forced evacuation of Cambodians from Phnom Penh into rural labour camps and murders at an execution site.
The pair have been accused of playing a critical role in the "Killing Fields" era, a genocide which left up to two million people dead in the late 1970s.
Former prisoner Keo Chandara, 63, said he helped exhume more than 12,100 skulls from eight mass graves at Kraing Ta Chan prison in Takeo province, around 80 kilometres (50 miles) south of Phnom Penh shortly after the regime fell in 1979.
"We did not excavate all the pits. I was ordered to dig the up pits... I did the excavation at those eight pits," he said.
The prosecution witness also gave graphic testimony of Khmer Rouge cadres torturing prisoners including a woman with metal pincers and then sulphuric acid.
"She was screaming and there were about 10 prisoners who were ordered to sit and watch the torture," Keo Chandara said.
"At that time they didn't take people through a court like this court. They just simply killed people," he added.
When the court resumed the genocide trial last month, the prosecution's first witness told judges that Khmer Rouge soldiers slit the throats of hundreds of inmates at the prison and ate their gall bladders.
The complex case against Nuon Chea and Khieu Samphan was split into a series of smaller trials in 2011 to try to obtain a faster verdict against the pair given their advanced ages and frail health.
They deny all charges against them and are appealing their life convictions.
Somewhere between 100,000-500,000 ethnic Cham Muslims and 20,000 Vietnamese were believed to have been killed during the regime's rule.
Led by "Brother Number One" Pol Pot, who died in 1998 without ever facing justice, the Khmer Rouge dismantled modern society in Cambodia in their quest for an agrarian Marxist utopia.