German police officers secure the site of an accident with a truck at a Christmas market on Breitscheidplatz square near the fashionable Kurfuerstendamm avenue in the west of Berlin, Dec 19, 2016. [Photo/Agencies]

The US State Department warned its citizens a month ago to be cautious visiting outdoor markets in Europe around Christmas.

The warning came after French authorities thwarted a plot linked to Islamic State.

Just last week, France renewed, for the fifth time, a state of emergency put in place after the Paris terror attacks of November last year. It gives the government exceptional powers over policing and public gatherings, but failed to prevent the deaths of 87 in Nice in July when a terrorist drove a truck into a crowd.

Britain meanwhile remains on "severe" alert, just one step short of "critical", reflecting the judgment of security officials that a terror attack is highly likely. Armed police patrols are a familiar sight in cities.

Spain remains at level 4 on a five-level scale, signifying that security personnel should be reinforced to the maximum.

In Italy, security was stepped up on Tuesday on the streets of major cities.

Many parts of Europe have suffered attacks, including, in2000, the Christmas market in Strasbourg, France that was the target of a thwarted plot by al Qaeda-linked terrorists.