Islamic State in Iraq and the Levant (Isil) has dispatched a ten-person squad to kill Russian tourists in Thailand, Russian security services have warned.

Thai police confirmed on Friday that they had received intelligence from Russia’s Federal Security Service (FSB), the main successor agency to the KGB, about 10 terrorists from Syria who reportedly entered Thailand in October.

The warning came to light in a leaked letter, marked “secret” and “urgent”, that began circulating on Thai social media on Thursday.

Thailand is a popular holiday destination for Russians, especially in the peak Christmas and New Year periods.

Phuket, Pattaya and Bangkok are all also popular with British visitors, meaning there is a high chance any terrorist attack there could also target Britons.

In the document, Special Branch police urged an intensification of security around “target areas that Russian authorities are concerned about” including venues associated with countries that have taken part in attacks on Isil in Syria.

The letter said 10 militants entered Thailand between October 15 and 31.

“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 FSB intelligence.

“Their purpose is to create bad incidents to effect Russians and Russia’s alliance with Thailand,” the letter said.

Russian officials have not commented on the reports, but Thai police said on Friday that the letter “is real”.

“So far it’s only intelligence news that still needs to be proved … we have no proof if they are here for real or not,” Songpol Wattanachai, a police spokesman, told reporters.

Russia launched air strikes in support of Syrian government forces in late September, prompting threats of revenge attacks from Isil, the self-styled terrorist group that controls larges parts of Syria and Iraq.

The terrorist group has previously claimed responsibility for the October 31 bombing of a Russian airliner leaving Sharm el-Sheikh, which killed 224 tourists.

On Wednesday the group released a video of what it said was the execution of an alleged “Russian spy.”

Security consultants have long feared that international Islamic terror groups could take advantage of the ease of travel to Thailand to dispatch attackers to strike at “soft targets” there.

“Thailand is what is called in military jargon a target-rich environment for terrorists, with foreign tourists, businesses and non-governmental organisations all here in large numbers,” said Anthony Davis, a veteran Bangkok-based security analyst with IHS-Jane’s.

In August, a bomb ripped through the Erawan shrine in central Bangkok, killing 20 Thais and ethnic Chinese visitors from other Asian countries. A network involving Muslim Uighurs from western China is alleged to have conducted the attack.

Thailand has so far avoided becoming a staging post for Islamic terror attacks against Western or now Russian targets. But experts have warned that terrorists operating under orders from Isil or inspired by the faction could conduct an atrocity similar to the beach rampage conducted by a lone gunman in Tunisia this year.