Ten Syrians with suspected links to the terrorist group have entered Thailand with their eyes on Russian targets, the memo says.
The internal police memo, dated November 27 and marked "urgent," has been seen by CNN and published in full by various Thai media outlets.
The memo says the Russian Federal Security Service provided the information to Thai authorities.
appears to have become a prime target for ISIS after the country's decision to involve itself in the Syrian civil war
Thai police: Security increased after Paris attacks
The 10 suspects, whose identities are not known, entered the country between October 15 and 31, and they headed for different parts of the country, the memo says.
Four traveled to Pattaya, 90 miles (145 kilometers) south of the capital city of Bangkok; two went to Phuket, about 520 miles southwest of Bangkok; and two remained in the capital, the memo says.
The whereabouts of the remaining two are unknown, according to the memo.
Thai deputy police spokesman Songpol Wattanachai said at a news conference Friday that the memo is genuine and that security in Thailand was increased after the recent terrorist attacks in Paris
Russia entered Syrian civil war two months ago
Russia entered the Syrian civil war on September 30, conducting airstrikes against ISIS
and other opponents of the Syrian President Bashar al-Assad
One month later, on October 31, a Russian plane crashed in Egypt's Sinai
, killing all 224 people aboard. Many international officials believe ISIS managed to smuggle a bomb onto the plane, causing the crash.
And then, on November 24, the Turkish military shot down a Russian warplane flying over Syria
after it allegedly strayed into Turkish airspace.
Both pilots ejected. One died, and the other was rescued.