US citizens travelling to Europe have been issued with new advice urging them to stay away from crowded places.
The warning has been drawn up in response to reports that members of al-Qaeda are planning to mount commando-style raids on cities in Europe.
The alleged plan involved sending teams of gunmen into crowded cities to kill Western civilians. No arrests have been made but a number of individuals are still under surveillance.
The suspects include British citizens of Pakistani origin and German citizens of Afghan origin.
An alert is less serious than a travel warning.
The travel guidance does not identify any specific targets and does not go as far as advising US citizens against travelling to Europe.
New York Times Foreign Correspondent John Burns said intelligence agencies had to publicise this threat, following mistakes that were made prior to terror attacks in London and the US.
The British Foreign Office has also warned that there was a 'high threat' of attacks in countries including France and Germany, rather than the 'general threat' previously identified.
British Home Secretary Theresa May urged the public to report any suspicious activity as police and security services try to disrupt terrorist activity.
Sweden announced on Friday that it had raised its threat alert to the highest level ever because of an increased threat of terror attacks.
