The US State Department issued a travel warning for American citizens going to Europe, citing the continued threat of terrorist attacks throughout the continent. The alert, which was released Monday (1 May) evening, will expire on 1 September 2017.

"While local governments continue counterterrorism operations, the Department nevertheless remains concerns about the potential for future terrorist attacks," the alert said. "US citizens should always be alert to the possibility that terrorist sympathizers or self-radicalised extremists may conduct attacks with little or no warning."

The alert mentioned incidents in France, Russia, Sweden and the UK as evidence that the Islamic State (Isis), al-Qaeda and other terrorist organisations "have the ability to plan and execute terrorist attacks in Europe."

The State Department warned that extremists focused on tourist locations, transportation centres, markets and local government facilities as potential attack targets. Hotels, restaurants, airports and other key locations remain as targets of possible attacks.

"Terrorists persist in employing a variety of tactics, including firearms, explosives, using vehicles as ramming devices and sharp-edged weapons that are difficult to detect prior to an attack," the alert continued. "If you are travelling between countries in Europe, please check the website of the US embassy or consulate in your destination city for any recent security messages."

The alert also urged US citizens to have an emergency plan of action ready and to register in the department's Smart Traveller Enrollment Programme (STEP).

In February, a US official told CNN that the terror threat in Europe was as high as it has ever been. The official said that recent estimates from Europe and the US show as many as 1,900 foreign fighters have returned to Europe after fighting in Iraq and Syria.

The State Department's previous travel alert for Europe was issued ahead of the winter holiday season and expired in February, Reuters reported.