Countries in Europe are on heightened alert after reports emerged that terror groups may launch attacks in capital cities on or before New Year's Eve.

The warning came from a "friendly'' intelligence service, which divulged the possibility of fresh terror attacks in the form of shooting or bomb attacks in capital cities across Europe, Vienna police said, according to NBC news.

The information reportedly made specific mention of the threat to shoppers and revellers in "crowded spaces'' and warned that the strike could take place between Christmas and New Year.

"In the days before Christmas, a warning was sent out by a friendly (intelligence) service to numerous European capitals, saying that it could come to an attack involving explosives or a shooting between Christmas and the New Year in crowded spaces,'' according to the statement released by police in Austria.

The European capitals have not been named, but security measures in Vienna have been increased after the city's police service said the detailed warning was made public.

Extra steps include surveillance in crowded spaces, "especially at events and traffic hubs" as well as intensive identity checks and higher alertness for objects which could carry explosives such as bags or "bicycle frames," it said, according to the report.

"The warnings could not be downplayed. Security measures have been increased to prevent a Paris-style copycat attack on an unknown location in Europe,'' according to the police.

Police have asked Austrians for their "understanding" of the need for more security controls.

"Several possible names of potential attackers were mentioned, which were checked, and the investigation based on (these checks) has so far yielded no concrete results," Vienna police said in a statement some six weeks after the Paris bombing, which left 130 people dead.

Meanwhile, a spokesman for the German Interior Ministry, said it would not comment on particular situations for "operational reasons."

"Germany is still in the crosshairs of jihadist terrorism," he said, adding the country had reviewed and adapted its security measures, where necessary, following the Paris attacks, according to the Daily Express.

Security has also been beefed up anew in other capital cities in a bid to thwart terror attacks.