The US state department today issued an alert to warn US tourists of the potential for attacks by al-Qaeda and other groups in Europe.
The travel advisory said public transportation systems and other tourism-related facilities could be targets.
"The State Department alerts US citizens to the potential for terrorist attacks in Europe," the statement said.
The alert said "current information suggests" al-Qaeda and affiliated groups were continuing to plan attacks.
A plot to stage coordinated attacks in Europe was disrupted in its early stages by drone strikes against militants in Pakistan, but it was not clear if the threat was fully erased, security sources said earlier in the week.
The US official, speaking on condition of anonymity because of the sensitivity of the issue, said the United States "is considering issuing a travel alert asking Americans to be vigilant as they travel in Europe given the most recent al Qaeda threat."
A state department official said the United States was consulting with European governments about the action.
Britain raised the terrorism threat level in its advice for citizens travelling to Germany and France to "high" from "general".
It left the threat level at home unchanged at "severe", meaning an attack is highly likely, and said it agreed with the US assessment for the continent as a whole.
Germany recently said it knew of information pointing to possible al Qaeda attacks in Europe and the United States, and intelligence sources said security agencies had disrupted plans by Pakistan-based militants for simultaneous strikes in London, as well as in major cities in France and Germany.
The plot involving al Qaeda and allied militants was in the early stages and would have involved groups of assailants taking and killing hostages, possibly along the lines of the 2008 raid in the Indian city of Mumbai in which 166 people died, the sources said.
Intelligence sources said an increase in strikes by unmanned U.S. drone aircraft on suspected militants in Pakistan in the past few weeks was part of Western efforts to thwart the plot.
Reuters