U.S. State Department on Sunday issued a travel alert to Americans over potential terrorist attacks in Europe.

"Current information suggests that al-Qaida and affiliated organizations continue to plan terrorist attacks," the State Department said in the travel warning.

"European governments have taken action to guard against a terrorist attack and some have spoken publicly about the heightened threat conditions," it said.

Last week intelligence officials in Britain said they intercepted a credible terror plot. The planned attack would reportedly have been similar to the deadly commando-style in Mumbai, India, two years ago, with other European cities, in France and Germany, also targeted.

"Terrorists may elect to use a variety of means and weapons and target both official and private interests. U.S. citizens are reminded of the potential for terrorists to attack public transportation systems and other tourist infrastructure," it said.

"Terrorists have targeted and attacked subway and rail systems, as well as aviation and maritime services," it added.

The department urged U.S. citizens to take every precaution to "be aware of their surroundings and to adopt appropriate safety measures to protect themselves when traveling."

The department also promised to work closely with European allies on the threat from international terrorism, including al- Qaida.

White House spokesman Nicholas Shapiro told reporters on Sunday that U.S. President Barack Obama has been closely following the potential threat.

"From the day we became aware of this latest plot, the president made clear we need to do everything possible to disrupt this plot and protect the American people," he said.

Under Secretary of State Patrick Kennedy told reporters that the U.S. has been following the situation for "at least several weeks", but he declined to get into the details of the intelligence.

"We're not saying don't travel to Europe. We are not saying don 't visit major tourist attractions or historic sites or monuments, " Kennedy said in a conference call.

Shortly after the United States issued the travel alert, Britain's Foreign Office also issued its latest warning on Sunday for its citizens travelling in France and Germany.

A spokeswoman of Foreign Office said: "We can confirm that the travel advice for France and Germany has been updated."

"Like other large European countries, they have a high threat of terrorism, which is reflected in our updated travel advice," she said.