"I deeply regret to learn of the Facebook post of H.E. Lee Hsien Loong, Prime Minister of Singapore, on May 31 expressing his condolences on the passing of General Prem Tinsulanonda in which he said ‘His time as PM coincided with the ASEAN members (then five of us) coming together to oppose Vietnam’s invasion of Cambodia and the Cambodian government that replaced the Khmer Rouge..’," Hun Sen wrote in his own Facebook post on Thursday. 

Tinsulanonda was PM and President of Thailand’s Privy Council.

In his Facebook post on May 31, PM Lee wrote: "Thailand was on the frontline, facing Vietnamese forces across its border with Cambodia. General Prem was resolute in not accepting this fait accompli, and worked with ASEAN partners to oppose the Vietnamese occupation in international forums. This prevented the military invasion and regime change from being legitimised." 

His statement reflects Singapore’s position then in support of the genocidal regime and the wish for its return to Cambodia, Hun Sen said.

"Singapore was the host of the tripartite meeting that led to the formation of the coalition government of the Democratic Kampuchea, which had prolonged the war and the suffering of Cambodian people for another 10 years. It was an act against the survival of the Cambodian people."

His statement is also an insult to the sacrifice of the Vietnamese military volunteers who helped liberate Cambodia from the genocidal regime, and it also reveals to the Singaporean people and the world that the leader of Singapore had indeed contributed to the massacre of Cambodian people, he wrote.

"Finally, I shall ask whether H.E. Lee Hsien Loong considers the trial of Khmer Rouge leaders legitimate."

Vietnamese Ministry of Foreign Affairs spokeswoman Le Thi Thu Hang said Tuesday that Vietnam "regrets" that Lee's statement did not reflect history objectively and created negative public opinion.

She pointed out that Vietnam's contributions and sacrifices in helping the Cambodian people end the Khmer Rouge' genocide are a fact and widely recognized.

The foreign ministry had discussed the issue with Singapore's Ministry of Foreign Affairs, she said. They have also sent a diplomatic note to Singapore Embassy in Hanoi. 

"I believe that Singapore understand clearly our message," Hang told the press on Thursday.

The U.N.-backed Extraordinary Chambers in the Courts of Cambodia found two leaders of the Khmer Rouge guilty of genocide last November, almost four decades after the regime which oversaw the ‘Killing Fields’ was overthrown.

Cambodian Defense Minister Tea Banh and People's Party legislator Hun Many have both rebutted PM Lee's remarks, saying they were untrue and could not be accepted, media reports said.

Minister Tea said he raised the matter with Singapore's counterpart at the Shangri-La summit last weekend and requested that Lee amend his statement.

The Khmer Rouge, led by Pol Pot, invaded Vietnam and killed tens of thousands of Vietnamese between 1975 and 1979 as part of its genocidal agenda. At the request of Cambodian revolutionaries, Vietnamese soldiers marched into Phnom Penh in January 1979 during a counteroffensive on the southwestern border to free the capital from the Khmer Rouge's clutches.