Japan on Monday became the latest country to issue a travel alert for Europe, cautioning its citizens to be wary of a possible attack.
Canada, the United States and Britain have all issued similar warnings. Japan's Foreign Ministry urged its citizens to be cautious on public transit and at tourist sites.
In Berlin, Germany's top security official, Thomas de Maiziere, said Monday that his country is assessing the threat level, but added that Germany saw no "concrete indication of impending attacks," CBC.ca reports.
Security services have warned of an al-Qaeda plans to send teams of gunmen into crowded places to kill civilians, like in Mumbai in 2008.
"I wouldn't imagine that a statement like this would've been done in a flaky way. This will have some kind of root in either an intelligence picture or on the back of what happened last week and the announcement last week that there was a thwarted attack in Europe and the UK. So, on that basis, we should take it seriously." said counter-terrorism expert Andy Hayman.
France is on high alert, just one step under its maximum level. Tourists have been evacuated from the Eiffel Tower twice in the last few weeks - and armed police patrol the streets, according to euronews.