Prime Minister Hun Sen defended the People’s Republic of Kampuchea’s controversial K5 Plan in a speech yesterday and accused self-exiled opposition leader Sam Rainsy of supporting the Khmer Rouge, just days after the self-exiled opposition leader accused the government of “crimes against humanity” in implementing it.

The K5 Plan was a strategy implemented between 1984 and 1987 to clear-cut, secure and mine a densely forested western border region to prevent Khmer Rouge forces from freely moving back and forth from Thailand.

Historians estimate that between 140,000 and 380,000 Cambodians were enlisted in a conscripted work effort, and thousands are thought to have died from malaria and land mines.

Hun Sen would have overseen at least some of the effort after becoming prime minister in 1985.

On Tuesday, Rainsy criticised Hun Sen and National Assembly President Heng Samrin on Facebook for filing lawsuits against him for “petty” issues, while ignoring his statement that they were involved in crimes against humanity in the execution of K5.

“K5-plan is the plan to gather people forcibly to be killed. Thousands of families in the forest stepped on landmines, were infected with malaria, starved, overworked or shot to death by the yuon,” it read, using an often derogatory term for Vietnamese, who backed the PRK.

Hun Sen hit back yesterday, maintaining that K5 had prevented the Khmer Rouge from returning to power. While not explicitly naming the opposition leader, he lobbed a thinly veiled taunt at Rainsy: “If you love the Khmer Rouge, come to be sentenced with them.”

Government spokesman Phay Siphan yesterday found even stronger words for the opposition leader.

“Sam Rainsy is a person who is indirectly a killer,” he said, arguing that Rainsy had colluded with the Khmer Rouge at the Thai-Cambodian border in the 1980s.

Asked about the forced labour, he said: “It’s wartime . . . Everyone [had] obligations to protect innocent people’s lives from the Khmer Rouge.”

Rainsy denied the accusations of collaboration yesterday, calling them a strategy to divert attention from crimes committed under K5.

He added that he was an editor of a publication in France that often criticised the Khmer Rouge, before daring Hun Sen and Samrin to sue him over his claims “so that we can expose the evidence that we have on both sides”.

Meanwhile, Youk Chhang, director of the Documentation Center of Cambodia, said the issue was more complex than presented by either side and pointed out that while K5 had legitimate objectives, many people suffered.