State Department warns about possible attacks in Europe

"U.S. citizens should exercise caution at holiday festivals, events, and outdoor markets," it said in a statement, adding that there was credible information that Islamic State, al Qaeda and their affiliates continued to plan attacks in Europe.

The department noted extremists had carried out attacks in Belgium, France, Germany and Turkey in the past year, and said it remains concerned about the potential for attacks throughout Europe.

"U.S. citizens should exercise vigilance when attending large holiday events, visiting tourist sites, using public transportation, and frequenting places of worship, restaurants, hotels, etc.," the statement said.