Lt. Col. Payao Thongsen, a senior investigator for Thailand's Department of Special Investigation, alleged Monday that 11 men who were arrested last week in northern Chiang Mai province were among 39 given ideological and combat training in a "neighboring country," providing details that made clear it was Cambodia.
"Why would we need to do this? Cambodia would receive absolutely no benefit from training these people," Cambodian government spokesman Khieu Kanharith said Tuesday. "Cambodia strongly rejects these allegations."
Payao's boss, DSI chief Tharit Pengdit, qualified the allegations on Tuesday, saying they were based on witness statements and needed to be verified.
"This is a sensitive issue involving our neighbor. We have to be especially careful," he said. "The legal process will continue, but we'll not give any more information. What has been said was by an officer at the operational level."
Prime Minister Abhisit Vejjajiva said he would not take any action until receiving advice from a government security agency.
Cambodia's relations with Thailand have been contentious for years.
The Thai allegations also raised the stakes domestically in an increasingly convoluted political battle that began in 2006, when Prime Minister Thaksin Shinawatra was ousted by a military coup for alleged corruption and disrespect toward Thailand's revered King Bhumibol Adulyadej.
The Red Shirts include many Thaksin supporters as well as activists opposed to military interference in politics.
Two months of protests in central Bangkok earlier this year by the Red Shirts demanding early elections degenerated into violence. About 90 people were killed in clashes before the army cleared the streets of demonstrators on May 19. Most top Red Shirt leaders have been detained on terrorism charges.
Dozens of bombings widely thought to be linked to the political strife have plagued Bangkok this year.
Payao said the 39 alleged trainees were part of a conspiracy to topple the monarchy, an allegation that his agency - the equivalent of the U.S. FBI - has made in the past against Red Shirts and their sympathizers.
The Red Shirt training included material meant to stir up hatred of the monarchy, he said in an interview with the government-owned MCOT television network.
Payao said the men were trained for three weeks at "a Cambodian army camp and they were trained by Cambodian soldiers," a report on MCOT's website said.
He said the men were "trained to know almost every kind of weapon," including assault rifles and grenade launchers, and were also shown the use of C4 plastic explosives.
He did not explain why Cambodia would be involved in such a conspiracy.
Thai-Cambodian relations, already strained by a dispute over territorial claims, worsened last year, with both countries withdrawing their ambassadors, after Cambodian leader Hun Sen made Thaksin an official adviser and hosted him like a VIP. Thaksin recently resigned that position, and the countries restored their ambassadors.
Payao said the alleged assassination targets included Abhisit and several other politicians, MCOT reported.
Jatuporn Prompan, a Red Shirt leader and member of Parliament, denied that his movement was part of any assassination plot, MCOT said.