The United States has updated its advice to travellers heading for Europe, warning of an increased terrorist threat over the Christmas and New Year holiday season.

Washington has a long-standing warning to its citizens to beware of extremist attacks in European cities, and in recent years it has expressly warned of dangers at festive events.

"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, a jihadist 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 the Al-Qaeda and the Islamic State 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 sympathisers or self-radicalised extremists may conduct attacks with little or no warning."