UN chief Ban Ki-moon said on Thursday that Cambodia's UN-backed war crimes court would decide whether to pursue more Khmer Rouge cadres, after Prime Minister Hun Sen ruled out a third trial of former regime members.
"This is the decision to be made by the court," Ban told reporters after touring a former prison and torture centre where he prayed for the souls of victims of the brutal regime.
"The United Nations will discuss this matter with international community members, particularly donors," Ban said.
Hun Sen told Ban Wednesday that a second Khmer Rouge war crimes trial due to start early next year would be the last and "case three is not allowed" because it could plunge the country back into civil war.
In its first trial the court in July sentenced former Khmer Rouge prison chief Duch, whose real name is Kaing Guek Eav, to 30 years in jail for overseeing the deaths of 15,000 people in the late 1970s.
Last month the court indicted four top regime leaders for war crimes, crimes against humanity and genocide in connection with the deaths of up to two million people from starvation, overwork and execution between 1975 and 1979.
The court is also investigating whether to open a third case against other former Khmer Rouge cadres, but has faced political interference.
Hun Sen was once a mid-level Khmer Rouge member himself before turning against the movement.
He has repeatedly warned that further prosecutions at the court could destabilise Cambodia, saying he would prefer to see the court fail than indict more suspects.
After seeing photos of the victims at Tuol Sleng prison, now a genocide museum, Ban made an emotional appeal for the Khmer Rouge's crimes not to go unpunished.
"We know it is difficult to relive this terrible chapter in your history," he said.
"But I want you to know: Your courage sends a powerful message to the world -- that there can be no impunity. That crimes of humanity shall not go unpunished," Ban said.