"I personally am not fully knowledgeable about everything, but I will try from the bottom of my heart to make sure that everything is fully revealed," said Khieu Samphan, the former head of state of the brutal regime.

"This is the most important moment for me and for my compatriots who are eager to know and understand what happened between 1975 and 1979."

Along with "Brother Number Two" Nuon Chea, ex-foreign minister Ieng Sary and his wife, one-time social affairs minister Ieng Thirith, Khieu Samphan faces charges including genocide, war crimes and crimes against humanity.

The elderly defendants are the most senior surviving members of a regime whose reign of terror led to the deaths of up to two million people from starvation, overwork or execution. The four deny the accusations against them.

AFP