The United Nations special representative for human rights in Cambodia has said it would be extremely sad if two Khmer Rouge leaders who surrendered at the weekend were not charged with crimes against humanity.

The official, Thomas Hammarberg, said people had to understand there should be no impunity in such matters.

He was speaking after the prime minister, Hun Sen, said there could be a return to civil war if the two men, Khieu Samphan and Nuon Chea, went on trial.

The men are due to meet Hun Sen today, and are said to be seeking fresh assurances that they won't be arrested.