WASHINGTON (REUTERS, NYTIMES) - The State Department on Monday (Nov 21) issued an alert to US citizens about a heightened risk of terrorist attacks throughout Europe, particularly during the holiday season.

"US citizens should exercise caution at holiday festivals, events, and outdoor markets," it said in a statement, adding that there was credible information that the Islamic State in Iraq and Syria (ISIS), 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.

"US 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.

The travel alert expires on Feb 20, 2017, it said.

The alert came as France on Monday said authorities had arrested seven men said to be planning a terrorist attack in the country.

Interior Minister Bernard Cazeneuve said investigators were looking into the possibility that the plot involved a "coordinated attack aimed to hit several sites simultaneously".

The men were arrested in the eastern city of Strasbourg and the Mediterranean port city of Marseille in an operation that began Sunday night, Cazeneuve said at a news conference, adding that the operation had "thwarted a terrorist attack that had been envisaged on our soil for a long time."

The arrests occurred a few days before the opening of the popular Christmas market in Strasbourg, which attracts more than 2 million visitors every year.