Tension and unease marked proceedings at the Khmer Rouge tribunal yesterday, as the Nuon Chea defence pressed expert witness Ysa Osman on the alleged involvement of current high-ranking government officials in the bloody suppression of Cham uprisings in 1975.

In September and October 1975, growing tensions between the ethnic Cham minority and the Khmer Rouge regime culminated in armed revolts in Kampong Cham’s Kroch Chhmar district.

The killing of ethnic Cham civilians in the suppression of these revolts figures into the alleged genocide of the predominantly Muslim minority, a key charge against co-defendants Nuon Chea and Khieu Samphan.

“There were two kinds of troops. First, the Cham people clashed with the troops from the Krouch Chhmar district and the troops from the district could not get into Svay Khleang and Koh Phal . . . There was another force sent from the sector level and they had cannons and artillery to shell the two villages and defeat them,” Osman said, when asked about which kinds of forces were deployed.

Pressed for detail, Osman said witness accounts say only that soldiers were mobilised from the south, “and that was Sector 21”, though Osman said it was uncertain who was in command.

Chea defender Victor Koppe then broached a contentious Human Rights Watch report that implicates Prime Minister Hun Sen – as a Sector 21 military official – in the quashing of the rebellion, and which the chamber had previously declined to add to the case file.

Osman confirmed that he was familiar with the report, as summarised in local media, and its contention that “the order to kill the Cham came from Sector 21, and certain high-level government officials were mentioned”.

Koppe then asked whether the prime minister was responsible for the suppression of the rebellion – as alleged in the HRW report – along with current National Assembly President Heng Samrin and Senator Ouk Bunchhoeun.

Osman maintained that he “did not mention any names of the government officials or that they led the forces to suppress and kill the Cham people”, but denied that the omission was out of fear, saying instead that “my researches have their limit”.

Continuing to pose highly sensitive questions about the three officials, Koppe repeatedly suggested that they could be answered in a closed session, though Osman ultimately refused “whether in closed or open session”.

“My apologies,” he added. “I cannot expand further.”

Turning to the judges, Koppe argued the witness must answer the question if it falls within his expertise, to which chamber ultimately agreed, albeit with caveats. Koppe then repeated his question for the last time.

“I’m putting it to you that Sector 21 and East Zone forces were solely responsible for the brutal suppression of this rebellion, and furthermore putting it to you that, among others, Mr Ouk Bunchhoeun, the number two of Sector 21 in 1975; Heng Samrin, the chief of the East Zone military staff; Sao Phim, the East Zone leader; and Hun Sen, regiment 55 commander in ’75 – that they were all responsible for these crimes against humanity.”

Osman allowed that interview subjects “made an indirect reference to those people, that they were part of the leadership in Sector 21. However, they did not mention that the four individuals, including Heng Samrin and Hun Sen, were implicated in the killing of the Cham people. And the four individuals that you mentioned, they are survivors and they are living now, and if you wish to get clarification on that, you should put such a submission to the chamber”.

Referring to the many rejected motions to call Samrin to testify, Koppe replied, “That’s my favourite hobby,” before dropping the line of questioning.