Europe’s Christmas markets are at risk of terror attacks, the US has warned

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. 

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.

In its advisory, the State Department pointed out that extremists had carried out attacks in Belgium, France, Germany and Turkey during the past year, and said it was still concerned about the potential for more attacks throughout Europe. 

It said "extremist sympathisers or self-radicalised extremists" might try to carry out attacks.

It added: "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 alert will stay in place until 20 February next year.