US citizens traveling to Sochi for next month’s Winter Olympics should be attentive of their personal security at the games following a series of terrorist attacks in another southern Russian city, the US State Department said Friday.

“There is no indication of a specific threat to US institutions or citizens, but US citizens should be aware of their personal surroundings and follow good security practices,” the State Department said in a travel advisory.

Two suicide bombers killed 34 people and injured more than 100 late last month in attacks on atrain station and a trolleybus in Volgograd, a city located about 600 miles (1,000 kilometers) from Sochi.

Russian authorities have since heightened security in Sochi, but the State Department travel advisory warned US citizens to use public transportation with caution, citing last months attacks as well as an October suicide bombing on a Volgograd bus that killed six people.

“US citizens are urged to remain vigilant and exercise good judgment and discretion when using any form of public transportation,” it said. “When traveling, US citizens may wish to provide a friend, family member or coworker a copy of their itinerary.”

Russia launched the largest security operation in Olympic history this week in a bid to prevent terrorism, particularly from the neighboring North Caucasus, a region plagued by an Islamist insurgency.

No group has claimed responsibility for the Volgograd attacks, but Russian authorities believe they were the work of Islamic militants from the North Caucasus.

The State Department said in its advisory that “large-scale public events such as the Olympics present an attractive target for terrorists” and that visitors should expect “increased police presence and enhanced security measures in and around the Olympic venues.”

“Russian authorities have indicated that they are taking appropriate security measures in Sochi,” the advisory said.

James Comey, director of the US Federal Bureau of Investigation (FBI), said Thursday that the agency is sending dozens of its personnel to Russia to help secure the Sochi Games and expressed confidence that Russia is up to the task.

“I think the Russian government understands the threat and is devoting the resources to address it,” Comey told Reuters.