Thai authorities have cleared three out of four Syrian nationals who reportedly overstayed their tourist visas and were suspected of being linked with the Islamic State jihadist group.
At least three countries -- Russia, Malaysia and Canada -- have issued travel alerts regarding travel to Thailand.
Nawfal Hassoun, 34, Mouayad Hayati, 29, and Abdul Aziz Arouk, 44, have been cleared after being questioned by police for more than three hours.
Another man, Hagop Kassabian, 57, has been found in Chaiyaphum's Chatturat district. His wife insists he holds a valid visa.
The names and photographs of the four were released on Saturday night by security officers, saying they have overstayed, after a leaked document reported that 10 Syrians linked to Islamic State had entered Thailand in October.
They had reportedly stayed in Bangkok, Pattaya, Phuket and other places and "could attack Russian interests" or those of allied countries.
The leaked document cited information from the Russian Federal Security Service sent to Thai Special Branch police. Security reports said Mr Hassoun entered through a checkpoint in Nong Khai on Aug 3 and that his visa expired on Oct 31. Mr Hayati entered through Suvarnabhumi airport on July 7. His visa expired on Oct 4.
Mr Arouk entered the country through an immigration checkpoint in Ranong on Jan 27 this year and his visa expired on April 26.
Mr Kassabian entered through Suvarnabhumi airport on May 16 and his visa expired on Aug 13.
Mr Hassoun and Mr Hayati reported to authorities Sunday after seeing their names among the four Syrians said to have overstayed their visas.
Mr Arouk was found at a house in the jurisdiction of Lumpini police station and was taken for questioning. Police in Chaiyaphum's Chatturat district visited Mr Kassabian's house Sunday after a tip-off and found him.
The three in Bangkok were questioned by senior police officers led by acting commissioner of Metropolitan Police Bureau Pol Lt Gen Sanit Mahathaworn at Lumpini police station.
The Russian ambassador to Thailand, Kirill Barsky, was present as an observer and gave information.
Pol Lt Gen Sanit said later police and immigration authorities questioned the three men for over three hours and believe they have no links with militant groups.
Mr Hassoun, who has a Thai wife, reported to police at Thung Mahamek station along with his wife, a native of Udon Thani.
The couple planned to open a restaurant in Bangkok. Mr Hayati reported to police at Lumpini station and insisted he had no involvement with any militant groups.
According to Pol Lt Gen Sanit, only Mr Arouk has been found overstaying and he would face legal action on the issue.
Under the immigration law, people who are found to have overstayed their visa will be charged and deported.
In Chaiyaphum, Mr Kassabian's wife Patcharin Piromkij, 38, told Chaiyaphum police they have lived together for more than 10 years and have three children.
She insisted her husband's visa is valid and they are ready to meet police to clear up any concerns.
Ms Patcharin said the family runs a business exporting fish and rice to Lebanon and some European countries.
Mr Kassabian had also appeared in several TV commercials with actors and actress including Jirayu "James" Tangsrisuk and Urassaya "Yaya" Sperbund.
Maj Gen Banpot Poonpian, spokesman for the Internal Security Operations Command, said Sunday there was no specific threat in the country and Thailand was not involved in any international conflict.
He also dismissed any connections between the Russian warning and the Ratchaprasong bombing. Maj Gen Banpot urged for calm and insisted security authorities were doing their best to handle public safety threats.
Meanwhile, Canada's authorities sent a message to its nationals saying there is a threat of terrorism throughout Southeast Asia, including Thailand.
"Some of you may have seen media reports that a group of 10 Syrians could have entered Thailand with plans to attack Russian citizens in popular tourist-resort cities as Pattaya, Phuket and Bangkok," the message said. "Maintain a high level of personal security awareness at all times as the security situation could deteriorate without notice."
It also suggested Canadians exercise caution, particularly in commercial and public establishments, including hotels, clubs, restaurants, bars, schools and places of worship, outdoor recreation events and tourist areas frequented by foreigners.
Malaysian ambassador to Thailand Datuk Nazirah Hussain advised Malaysians travelling to Thailand to be vigilant and alert, according to a report at The Star Online. "Contact the embassy for any information or assistance," she said.
The head of Russia's consumer protection agency is advising Russians not to travel out of the country over the 10-day New Year's holiday because of an array of health risks. Security risks already have been cited as reasons not to travel to popular vacation spots such as Egypt, Turkey and Thailand, an Associated Press report said.