Terror threat to Christmas markets
Artists dressed as Christmas Angels pose at the opening of the Christmas market at Gendarmenmarkt square in Berlin, GermanyREUTERS/Hannibal Hanschke

The US State Department is warning its citizens in Europe to be alert over the Christmas period because of a "heightened risk" of terror attacks.

It said it had "credible information" that Islamic State, al Qaeda and other militant groups were continuing to plan attacks in Europe.

"US citizens should exercise caution at holiday festivals, events, and outdoor markets," it added.

The advisory comes a day after France said it had arrested seven terror suspects in Strasbourg and Marseilles who had been plotting an attack for months.

According to Sky News, officials did not reveal the potential targets, but the arrests rattled nerves in Strasbourg, where a famed Christmas market opens in a few days.

The market, which attracts tourists from across the world, was the target of a failed extremist plot in 2000 by Algerian and French militants who had trained in Afghanistan.

Top French security expert Jean-Charles Brisard, director of the French Center for Analysis of Terrorism, told Newsweek on Monday that the latest thwarted plot showed ISIS networks in France "were planning" and in spite of tougher government measures, "their capacity has not been affected, nor their intention to target Western countries."

One EU security official, who spoke to Newsweek on condition of anonymity: "Will we have another Bataclan? I would not be surprised; probably not on that scale, but at least a plot [of that size]."

The threat levels in many Western European nations remain high. In Britain the warning level regarding an attack remains at "severe." MI5 chief Andrew Parker said earlier this month that security services had foiled 12 plots in the past three years alone. The American warning will only add to Europeans' fears that they have not seen the last of the bloodshed in the continent's cities.

The alert does not mention any specific countries as being particularly at risk but notes that the last year has seen attacks in Belgium, France, Germany, and Turkey. The warning makes no mention of Britain.

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