Jailed Khmer Rouge prison chief Duch cried several times Tuesday as he toured a notorious site of mass graves on the outskirts of Phnom Penh, a tribunal official said.

Duch, who has been charged with crimes against humanity, toured the Choeung Ek "killing fields," with tribunal judges, witnesses and lawyers, as part of proceedings against him.

"We saw Duch express his regret for the dead victims, and Duch's tears fell two or three times," tribunal spokesman Reach Sambath said after the tour. "Before he left, he prayed for the dead victims, at the stupa of skulls, and he cried."

Duch and witnesses exchanged questions and answers in front of prosecutors, investigating judges and lawyers, Reach Sambath said, but he declined to give further details.

Thousands of Cambodians were buried at Choeung Ek after torture and execution, allegedly under the supervision of Duch, whose real name is Kaing Khek Iev, during the Khmer Rouge regime.

Skulls of the dead are piled within a glass-cased stupa at the "killing fields," now a popular tourist site.