Moscow’s top intelligence agency has warned that 10 Syrians linked to ISIL have entered Thailand to target Russians, the kingdom’s police said Friday.

Moscow's top intelligence agency has warned that 10 Syrians linked to the Takfiri group, ISIL (so-called Islamic State in Iraq and Levant) have entered Thailand to target Russians, the kingdom's 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) had told Thai police that the group of Syrians entered the country between October 15 and 31 potentially 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.

But Thailand's national police chief Jakthip Chaijinda told reporters the "document was real".

Since the middle of October more than 200 Syrians have entered Thailand.

"Twenty remain here," Jakthip said, adding he did not know if the 10 people referenced in the letter were counted among the 200 who have entered this autumn.