The US State Department has issued a travel warning for its citizens going to Europe, citing the continued threat of terror attacks.

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

The warning mentioned recent attacks in France -- which heads to the polls next week to choose a new president -- Russia, Sweden and the UK, noting that extremists tend to focus on tourist hot spots.

A US official told CNN in February that the terror threat in Europe was as high as it's ever been.

Recent estimates from Europe and US say as many as 1,900 foreign fighters have returned to Europe after stints in Iraq and Syria, the official said. Some remain committed to terror groups like ISIS, but others returned disenchanted.

The alert expires September 1.