The US administration on Sunday warned Americans of potential terrorist threats in Europe and urged them to be vigilant in public places, including tourist spots and transportation hubs.
A State Department travel alert advises US citizens living or travelling in Europe to take more precautions. The alert is one step below a formal travel warning advising.
"Current information suggests that al Qaeda and affiliated organisations continue to plan terrorist attacks," it said. "European governments have taken action to guard against a terrorist ... and some have spoken publicly about the heightened threat ..."
Britain's Foreign Office upgraded its travel advice for France and Germany, warning Britons going to those countries that the threat of terrorism there is high. Before Sunday's change, the government's travel advice for France and Germany was that the threat from terror attacks there was "general."