Phnom Penh - A witness told Cambodia's UN-backed court Wednesday that Khmer Rouge soldiers slit prisoners' throats and ate their gall bladders during the 1970s, as the genocide trial of the two most senior surviving leaders resumed.Nuon Chea, aged 88, known as "Brother Number Two", and former head of state Khieu Samphan, aged 83, face charges over the killing of ethnic Vietnamese and Muslim minorities, forced marriage and rape during the 1975-1979 regime that left up to two million people dead.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 one execution site.The genocide trial, which began last July, has faced repeated delays due to boycotts by the defendants' lawyers and most recently because of the brief hospitalisation of Khieu Samphan.He was well enough to attend the hearing on Wednesday alongside his defence team, while Nuon Chea followed the proceedings from a holding cell due to dizziness.The prosecution's first witness, Meas Sokha, told the court that he saw Khmer Rouge soldiers kill hundreds of inmates at Kraing Ta Chan prison in Takeo province, around 80km south of Phnom Penh, where he was detained along with 11 other family members."While I was tending cows and buffaloes, I could see how prisoners were killed. Most of them had their throats slit," said the 55-year-old."Two [Khmer Rouge guards] would hold a prisoner tight and another would slit the throat of the prisoner," he said, adding that the Khmer Rouge played music through a loudspeaker "to hide the sounds of the killing".He said at least 20 prisoners were killed per day after a week of starvation at the prison, where he was held for more than two years from 1976. His father and younger siblings also died at the jail.The witness added that soldiers killed small children by throwing them against a tree before dropping their bodies into the mass grave at the prison.He told the court that Khmer Rouge cadres would eat the gall bladders of executed prisoners after drying them in the sun."Whenever there were killings, the guards would drink wine together with gall bladders," said Meas Sokha. "I knew these gall bladders were from humans. There were many gall bladders dried in the sun near the fence."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, both of whom are elderly and frail.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.