A former Khmer Rouge leader wept while visiting a killing field as part of a genocide tribunal's attempt to piece together what happened during the regime's brutal rule, an official said Tuesday.

Kaing Guek Eav, also known as Duch, spent 3½ hours at a notorious mass burial site south of Cambodia's capital of Phnom Penh on Tuesday.

Duch, 65, is one of five former Khmer Rouge leaders charged with crimes against humanity and brought before the joint Cambodia-UN genocide tribunal for their role in the mass deaths.

An estimated 1.7 million died during the Communist regime's 1975-1979 rule, when they cut off contact with the outside world and forced people to work in an agricultural collective. Many died from starvation and disease, with thousands more tortured and executed.

Taking Duch back to the notorious killing field was part of the tribunal's investigative process to try to figure out what happened during the regime's brutal rule.

Tuesday's visit was closed to the public and media, but an official later reported on it.

Tribunal spokesman Reach Sambath said that Duch wept during the visit as he "explained what happened … when he was the chief of S-21."

"We noticed that he was feeling pity, tears were rolling down his face two or three times," Sambath said.

Tribunal visits tree where children executed

Duch faces charges in connection with his role three decades ago as leader of the infamous Security Prison 21, or S-21, a high school in Cambodia's capital of Phnom Penh that was converted into a prison and torture centre. He was taken into custody by the UN-assisted tribunal last year pending trial.

The spokesman said Duch was particularly moved when he stood before a tree with a sign describing how executioners killed children by bashing their heads against its trunk.

Other displays among the shallow mass graves include skeletal remains and ragged clothes.

About 16,000 men, women and children held at S-21 were killed at Choeung Ek, now a memorial site popular with tourists.

At the end of Duch's tour, he clasped his hands together in prayer and cried again in front of a glass-fronted Buddhist stupa filled with nearly 9,000 skulls, some of which bear clear evidence of hammers, hoes and bullets, Sambath said.

Visit to genocide museum slated for Wednesday

Duch was driven in a heavily guarded convoy from the tribunal's detention centre to Choeung Ek, some 19 kilometres south of Phnom Penh.

About 80 people, including judges, lawyers and representatives of victims and witnesses, were present for the re-enactment, Sambath said.

Among the witnesses were four former staff members of S-21, he said.

Duch is to visit S-21, now the Tuol Sleng genocide museum, on Wednesday.

Only 14 of the thousands of those jailed in the prison during the Khmer Rouge reign are believed to have survived.

The long-delayed genocide trials may start later this year. There are concerns that the group's surviving leaders could die before being brought before justice, as the movement's chief, Pol Pot, did in 1998.

With files from the Associated Press