Jailed Khmer Rouge prison chief Duch is scheduled to take tribunal judges on a tour of the "killing fields" outside Phnom Penh Tuesday, as part of an investigation of war crimes charges against him.

"He is the key person in the matter of S-21, so if [judges] go to visit those places in order to see the practical situation, they have a right to summon Duch to go along with them," said Hisham Mousar, a tribunal monitor for the rights group Adhoc.

Duch, whose real name is Kaing Khek Iev, 65, is expected to visit the Choeung Ek site, where executed Cambodians where buried in mass graves during the Khmer Rouge regime, on Tuesday and to visit Tuol Sleng prison, where he was chief, on Wednesday.

The tours of Choeung Ek and Tuol Sleng are a regular part of the tribunal system and meant to make matters more clear for the investigating judges.