FBI Director James Comey says the Olympics in Sochi next month will be "particularly challenging" to secure.
Comey added that "securing any Olympics is an enormous task, but Sochi's proximity to areas of unrest and sources of terrorist threat" will be demanding.
U.S. law enforcement and other federal personnel are in Russia to provide security advice and support.
The United States will have one of the largest contingents of athletes and officials at the 17-day games, and Comey said American security personnel will be in place both in Sochi and in Moscow.
There has been heightened concern about security following twin bombings on public transit in the southern city of Volgograd-- about 400 miles from Sochi-- at the end of last month.
Russian officials have promised a safe, welcoming environment for the estimated 6,000 athletes from 85 countries, plus 1,650 paralympians from 45 countries in a separate competition later in the month.
No top U.S. government official will attend the games. It will be the first time since 2000 that a U.S. president, former president, first lady or vice president will not be going.