The United Nations-backed tribunal in Cambodia has heard how up to 200 children were among the thousands of prisoners processed and killed at the Khmer Rouge's most infamous detention centre.

Sous Thy, a former guard and bookkeeper at the S-21 prison, says between 100 and 200 children were separated from their parents who were brought to the camp during the Khmer Rouge rule.

Although prison records were kept for adults, Sous Thy says no such records were kept of the children.

They were bludgeoned to death in the Killing Fields on the outskirts of the capital.

This is the first time that a number has been put on the children killed at the camp, and the prison was just one of hundreds used by the communist government in the mid-1970s.

The court has also heard that prisoners were often drawn of their blood before being killed.