Opposition leader Sam Rainsy yesterday repeated allegations that Foreign Minister Hor Namhong was responsible for collaborating with his Khmer Rouge captors at the Boeung Trabek prison camp between 1975 and 1979, leading to the murder and torture of numerous inmates.

Rainsy, who is in the Philippines, postponed a planned return to Cambodia on Monday after a warrant was issued for his arrest over a defamation ruling stemming from similar comments he made in 2008, for which he could face a two-year prison term.

The Cambodia National Rescue Party president likened Namhong’s role at the Boeung Trabek camp to that of a “kapo” – prisoners in Nazi concentration camps selected by the SS to supervise their fellow inmates, who were known for their brutality towards other prisoners – a comparison he has previously made in relation to the allegations.

Following a suggestion this week from media mogul and political fixer Soy Sopheap that Rainsy may be able to avoid or reduce his two-year sentence if he penned a letter of apology to Namhong that recognised the ruling Cambodian People’s Party as the “national liberator from the Khmer Rouge”, Rainsy was unrepentant.

“It is Hor Namhong who should apologise to the souls and the families of the victims who got killed by the Khmer Rouge at the Boeung Trabek prison in the late 1970s because he acted as a Kapo and denounced them to their torturers and killers,” he said in an email.

Rainsy on trial: view an interactive timeline of the opposition leader's ongoing legal battles

Rainsy returned to Cambodia from self-imposed exile in France ahead of the 2013 election and it was thought that he had received a pardon for the defamation ruling until a lawyer for Namhong had the court issue the warrant on Friday.

Speaking to reporters after a meeting with dozens of foreign ambassadors and United Nations representatives yesterday, Namhong claimed he and Prime Minister Hun Sen had forgotten about the case and that his lawyer had acted independently.

“I did not instruct my lawyer to do anything . . . there was no directive from myself or anyone else, not from Samdech Hun Sen. We forgot about it [the case], and had no idea what my lawyer was doing until the arrest warrant was issued, so there was no political pressure,” he said.

Kar Savuth, the lawyer in question, could not be reached yesterday.

Sok Eysan, a ruling party spokesman, yesterday said that the CPP would pursue further defamation charges against Rainsy in the near future over a post on Facebook this week insinuating that members of the party had accused the late King Father Norodom Sihanouk of being a traitor and attempted to have him sentenced to death in the 1980s.

“This is more incitement and defamation, which is a very serious attempt to stir people to rebel against the CPP,” Eysan said. “He must be responsible before the law in one more case.”

But CNRP lawmaker Eng Chhay Eang played down the significance of Rainsy’s claim, saying there was “no serious meaning” to the post, which was a “reflection of public opinion”.

While Chhay Eang argued that it was unconstitutional that Rainsy’s parliamentary immunity was stripped by the National Assembly under Article 77, Namhong said Rainsy had failed to forward the case to the Supreme Court level after the Appeal Court decision.

In 2011, anti-secrecy group Wikileaks published a 2002 US Embassy cable that said when Namhong returned to Cambodia in 1975, he “became head of the Beng Trabek [sic] camp and he and his wife collaborated in the killing of many prisoners”.

However, another cable released by Wikileaks from 2008 suggested that Rainsy was engaging in a “high-visibility slur campaign” with little evidence to back up his claims.