IS agents in Thailand 'to hit Russian interests'
The letter was signed by the deputy head of Thailand's special branch and marked "top secret" and "urgent".
It said Moscow's Federal Security Service [FSB] had informed Thai police that ten Syrians entered the country between 15 and 31 October.
|The "top secret" memo was leaked to the press [Twitter]|
"[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, Russia's security agency.
"Their purpose is to create bad incidents to affect Russians and Russia's alliance with Thailand," the letter said, without naming the suspects.
See Also: Russia 'expanding' second air base in Syria
More than 1.6 million Russian tourists visited Thailand in 2014, the largest number of all 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.
But Thailand's national police chief Jakthip Chaijinda told reporters the "document was real".
A second deputy police spokesman, Krissana Phattanacharoen, also confirmed "the content of the letter is genuine", but played down a specific threat to the country.
Since mid October more than 200 Syrians have entered Thailand.
"Twenty remain here," Jakthip said, adding he did not know if the ten people referred to in the letter were included among the 200 who have entered this autumn.
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.
On Wednesday, IS released a video showing the murder of a Russian man, allegedly for being a "spy" for Moscow.
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.
But two men from China's Muslim Uighur minority have been charged over the blast.
That has heightened speculation of a link with supporters of the Uighurs, who say they face persecution in their homeland.
Thailand had forcibly deported 109 Uighurs to China just weeks before the bombing.