Bangkok - Moscow's top intelligence agency has warned that 10 Syrians linked to the Islamic State group have entered the kingdom to target Russians, Thai police said Friday.
A leaked letter, marked "top secret" and "urgent" and signed by the deputy head of Thailand's special branch, was widely circulated in local media late Thursday.
It said Moscow's Federal Security Service (FSB) has told Thai police that the group of Syrians entered the country between October 15 and 31 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.
More than 1.6 million Russian tourists visited Thailand in 2014, the largest number from European nations.
Arrivals from Russia spike during the Christmas and New Year holiday season.
The Russian embassy in Bangkok would not immediately comment on the letter.
Songpol Wattanachai, a deputy spokesman for the Thai police, told reporters "the letter is real".
But he added: "We have no proof if they are here or not."
A second deputy police spokesman, Krissana Phattanacharoen, confirmed "the content of (the) letter is genuine," 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, referring to the potential danger posed by the Islamic State group.
Thailand is in its peak holiday season, during which international arrivals surge, bringing huge sums of money to the economy.
Confirmation that Islamic State jihadists have entered the country would likely send jitters through the tourist industry, especially in busy resort areas such as Phuket and Pattaya - both popular with Russians.
Pattaya police urged tourists not be alarmed by the reports, although they said security will be beefed up.
Russia launched air strikes against IS targets in Syria in September. A month later, a Russian passenger plane was downed by a bomb over the Sinai desert in Egypt killing 224 people, mainly Russian holidaymakers.
Islamic State jihadists later claimed responsibility for the bombing as well as the November 13 attacks on Paris that killed 130 people. The brazen attacks have further raised global alarm over the possibility of assaults by IS gunmen and bombers.
Thailand was hit by a bomb in August that left 20 people dead at a shrine in downtown Bangkok, rattling the tourist industry.
Mystery still shrouds the motive for the unclaimed attack, whose aftermath saw Thai police release contradictory and confusing information.
Thailand has not fallen victim to a mass casualty attack by Islamic extremists in recent years -- although many of its Southeast Asian neighbours have.