BANGKOK - Fears are growing over the fate of three Thai anti-monarchy fugitives said to have been turned over by Hanoi to Bangkok on May 8 but whose whereabouts the Thai government denies knowledge.

Their disappearance comes just five months after the badly mutilated bodies of two other anti-monarchy activists were found in the Mekong River.

Political activists Chucheep Chiwasut, Siam Theerawut and Kritsana Thapthai were detained by Vietnamese authorities early this year and sent back to Thailand on May 8, according to an activist group - Thai Alliance for Human Rights.

But Thai authorities, including deputy prime minister Prawit Wongsuwan, say they do not know of any repatriation.

Reached on Monday (May 13), Thailand's foreign ministry spokesman Busadee Santipitaks said: "I have seen the news reports and have no further information."

Police Lieutenant General Sarawut Kanphanit, the commander of Thailand's special branch unit, described the attempts to link the kingdom to the three missing fugitives as rumour that is being used to discredit Thailand as a brutal country.

He told The Straits Times: "This is just a rumour. No reliable source can be identified from both countries. We haven't had any report of these people and there is not report of their arrest in Vietnam or any other country."

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The Straits Times has reached out to Vietnam's foreign ministry as well.

Mr Siam's mother Kanya Theerawut lodged a petition with the Vietnamese embassy on Monday afternoon asking for information about her son.

"I am worried for his life," she said, her voice cracking up as she addressed reporters in front of the embassy in Bangkok.

"Our country has laws. I want them to follow the law. If he was really brought to Thailand, where is he? And was he really caught for using a fake passport in Vietnam?"

Their disappearance has spread alarm because three political activists who broadcast anti-monarchy and anti-junta messages from their hideout in Laos had similarly gone missing on Dec 12 last year (2018).

Later that month, two bodies that had been cut open and stuffed with concrete were found in the Mekong River that forms the border between Laos and Thailand.

Their "hands and feet were bound and their faces smashed beyond recognition," said Human Rights Watch.

DNA tests later confirmed that the bodies belonged to Mr Kraidej Luelert and Mr Chatchan Buphawan, aides of prominent anti-monarchist Surachai Danwattananusorn, who had fled Thailand in 2014 shortly before the current military government was installed by a coup.

Mr Surachai remains missing and is thought to be dead.

Thailand is a constitutional monarchy where insulting or defaming the king, queen, regent or heir apparent is punishable by up to 15 years jail on each count.

But there have been no fresh lese majeste prosecutions since last year (2018), something attributed to the intervention of newly crowned King Maha Vajiralongkorn.

Still, the Thai government has accused Mr Chucheep - whose nickname is Uncle Sanamluang - of heading a secessionist movement from Laos.

Deputy Prime Minister Prawit said last September (2018): "We now ask (Laos) not to help them anymore."

Mr Chucheep and his two aides moved from Laos to Vietnam shortly after the murder of their colleagues.

Mr Sunai Phasuk, senior researcher for Human Rights Watch told The Straits Times: "The reaction of both the Thai and Vietnamese authorities gives us no confidence at all about the safety of the three missing activists."

Activists have also speculated about the nature of possible cooperation between Vietnam and Thailand on the extradition of their respective dissidents.

In January (2019), Vietnamese journalist Truong Duy Nhat disappeared in Bangkok one day after applying for asylum at the United Nations High Commissioner for Refugees there.

In March, he was found in a Hanoi jail.

"We were afraid there would be a quid pro quo," said Mr Sunai. "But these are serious allegations... they need to have serious investigation and clear explanation."

Mr Siam's father Sathian Thirawut remains hopeful that the Vietnamese embassy can give some answers.

"If they don't, I will have to find them myself. I have to go to the immigration police, or maybe Vietnam," he said.