Moscow's top intelligence agency has warned that at least 10 Syrians linked to the Islamic State of Iraq and Syria (ISIS) have entered Thailand to target Russians and the country's interests there, Thai police have said.
Local media on Thursday quoted from a leaked letter, marked "top secret" and "urgent" and signed by the deputy head of Thailand's special branch that was widely circulated.
Moscow's Federal Security Service (FSB) has told Thai police that the group of Syrians entered the country between 15 and 31 October aiming to target Russian interests.
"They (the Syrians) travelled separately. Four went to Pattaya, two to Phuket, two to Bangkok and the other two to (an) unknown location," the letter said, citing information from the FSB.
"Their purpose is to create bad incidents to affect Russians and Russia's alliance with Thailand," the letter said, without naming the suspects.
Thailand is a major holiday destination for Russian tourists, particularly during the peak Christmas and New Year holidays. More than 1.6 million Russian tourists visited Thailand in 2014, the largest number from a European nation.
Arrivals from Russia spike during the Christmas and New Year holiday season.
Songpol Wattanachai, a junior spokesperson for the Thai police, told reporters, "the letter is real," but he said, ''We have no proof if they are here or not."
"Please be confident - we won't disregard the intelligence," he added.
"The content of (the) letter is genuine," another deputy police spokesperson Krissana Phattanacharoen said, but played down a specific threat to the country.
"Yes it's a threat, not only to my country but also to other countries as well," he added. 'Tourism Jitters.'
Thai police sought to reassure tourists flocking to Phuket and Pattaya - both popular with Russians - that security will be beefed up and that they need not be alarmed by the reports.
Thailand is in its peak holiday season, during the international festival season, when tourist arrivals, especially from Russia, surge.
Confirmation that ISIS terrorists have entered the country would likely send jitters through the tourist industry, especially in busy resort areas such as Phuket and Pattaya.
Russia is extremely concerned over ISIS attacks after the ISIS planted a bomb in a Russian passenger plane and downed it over the Sinai desert in Egypt, killing all 224 people, mainly Russian holidaymakers.
The incident happened a month after Russia launched air strikes against ISIS targets in Syria in September.
Islamic State jihadists also massacred 130 people in Paris on 13 November, further raising global alarm over the possibility of assaults by IS gunmen and bombers.