Story highlights The alert mentioned recent attacks in France, Russia, Sweden and the UK.

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

(CNN) The US State Department has issued a travel alert 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 alert 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.

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