Japan and Sweden have become the latest countries to issue travel alerts for Europe, cautioning their citizens to be wary of a possible attack.

Canada, the United States and the United Kingdom have all issued similar warnings.

Japan's Foreign Ministry urged its citizens to be cautious on public transit and at tourist sites.

French soldiers under the Eiffel Tower in Paris on Sunday, as the British Foreign Office mentioned France and Germany in a travel advisory about possible militant attacks. (Laurent Cipriani/Associated Press)

Authorities are concerned that militants may be planning co-ordinated attacks in Europe similar to the assault by 10 gunmen on targets across Mumbai, India's financial hub, in 2008.

In Berlin, Germany's top security official, Thomas de Maiziere, said Monday that his country is assessing the threat level but has seen no "concrete indication of impending attacks."

On Sunday, the British Foreign Office mentioned France and Germany in an upgraded travel advisory for its citizens. Sweden's warning did not single out any particular countries.

While Canadians are not being told to avoid travelling in Europe, they are being advised to be vigilant, monitor local news reports and follow the advice of local authorities.

Public Safety Minister Vic Toews said Canadian officials are working closely with those in the U.S. and in Europe.

"We don't have any specific advice to give beyond that at this time, but I can assure you that we are monitoring matters very, very closely," Toews said.

The warnings from the U.S. and Europe are vague as well.

Britain's former leading anti-terrorist police commissioner, Andy Hayman, said this could be a problem.

"It's a very general alert; it's non-specific," he said. "And it could actually have the detrimental effect of rather than raising people's awareness — because it's so general — people just ignore it."