Washington has a long-standing warning to its citizens to beware extremist attacks in European cities, and in recent years it has expressly warned of dangers at festive events, AFP reported.
"Last year, mass casualty attacks occurred at a Christmas market in Berlin, Germany in December and a nightclub in Istanbul, Turkey on New Year's Eve," the latest warning noted.
Last year an attacker crashed a truck into Christmas merrymakers in a Berlin market, killing 12 people, while a New Year's Eve attack in Istanbul left 39 dead.
The State Department warned that recent attacks in Britain, Finland, France, Russia, Spain and Sweden show that both Al-Qaeda and the Daesh (also known as ISIL or ISIS) group remain active and capable.
"While local governments continue counterterrorism operations, the department remains concerned about the potential for future terrorist attacks," the statement said.
"US citizens should always be alert to the possibility that terrorist sympathizers or self-radicalized extremists may conduct attacks with little or no warning."