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"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.