The Department of State issued the travel alert, warning US citizens of the “heightened risk of terrorist attacks throughout Europe, particularly during the holiday season”.

It adds that “recent, widely-reported incidents in France, Russia, Sweden, the United Kingdom, Spain, and Finland demonstrate that the Islamic State of Iraq and ash-Sham (Isis or Da’esh), al Qaeda, and their affiliates have the ability to plan and execute terrorist attacks in Europe”, the Independent 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. Terrorist sympathizers or self-radicalized extremists may conduct attacks with little or no warning.”

The Australian government has also reportedly warned its citizens visiting the UK of a heightened threat of terrorist attack in the UK”.

“There are two key sources of this threat, it adds. “(i) groups and individuals motivated by the current conflict in Iraq and Syria (’international terrorism’) and (ii) groups and individuals motivated by the status of Northern Ireland”.

Canadian tourists visiting Britain have also been warned to “exercise a high degree of caution at all times, monitor local media and follow the instructions of local authorities.”