WASHINGTON • 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 longstanding warning to its citizens to beware of extremist attacks in European cities, and in recent years, it has expressly warned of danger 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 on Thursday.

Last year, a militant crashed a truck into a Christmas market in the heart of Berlin, killing 12 people, while a New Year's Eve attack in Istanbul left 39 dead.

The US State Department warned that recent attacks in Britain, Finland, France, Russia, Spain and Sweden show that both Al-Qaeda and the Islamic State in Iraq and Syria remain active and capable.

"While local governments continue counter-terrorism operations, the department remains concerned about the potential for future terrorist attacks," said the statement.

"US citizens should always be alert to the possibility that terrorist sympathisers or self-radicalised extremists may conduct attacks with little or no warning."

AGENCE FRANCE-PRESSE