The news was revealed by Thai police who spoke after a leaked letter, marked "top secret" and "urgent" and signed by the deputy head of Thailand`s special branch, was widely seen by local media.

The letter said Moscow`s intelligence service had revealed a group of Syrians arrived in Thailand between 15 and 31 October 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.

"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 - with Christmas and New Year being the most popular time - the largest number from any country in Europe.

Thailand`s police boss Jakthip Chaijinda confirmed the letter "was real" and said more than 200 Syrians had entered Thailand over the autumn.

The peak holiday period for Thailand is coming up, bringing the hope of huge sums of money from tourism.

However, fears that IS Islamists may be planning an attack is likely to send fears through the tourist industry, particularly in busy resort areas such as Phuket and Pattaya, which are both popular with Russians.

Pattaya police, who say they have increased security in the area, have urged tourists not to be concerned by the reports.

Thailand`s capital, Bangkok, was the target of a bombing in August in which 20 people were killed.

However, that was reportedly unconnected with IS supporters.

Russia decided to support Syria`s government in September, launching air strikes against IS targets in Syria.

A month later, a Russian passenger plane carrying 224 people travelling back from the resort of Sharm el Sheikh was brought down by a bomb over the Sinai desert in Egypt.

All on board were killed.

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