The U.S. government says there's no indication the United States or its citizens are a target in the latest Al Qaeda plot that triggered a blanket travel alert for Americans in Europe.
White House Press Secretary Robert Gibbs said Monday that the State Department alert "deals with Europe and isn't related to the United States."
An FBI/Homeland Security intelligence bulletin obtained Monday by The Associated Press also said the latest threat did not appear to involve the United States.
"At this time, there is no indication that the reported threat is directed specifically toward the United States, its citizens, or infrastructure; however, we assess that Al Qaeda and its affiliates continue to plot against the Homeland and U.S. allies," the bulletin said.
But officials are nevertheless urging Americans abroad to be on the lookout for suspicious activity, as new information emerges about the terror plot that prompted the unusual alert from the State Department.
Two intelligence sources told Fox News that possible targets include several tourist attractions across Paris and Berlin. ABC News reported Monday that at least five European airports could be targeted as well.
Lt. Col. Pamela Cook, a spokeswoman at Ramstein Air Base in Germany, told Fox News that the base conducted a 24-hour security exercise between Friday and Saturday nights in response to the threats, though they were under no region-wide directive to do so. Cook said the exercise involved a curfew and heightened security on and near the base -- service members were told not to wear their uniforms off base.
Officials are warning about the possibility that terrorists -- under the leadership of Usama bin Laden -- are trying to execute a Mumbai-style rampage in western Europe.
The list of possible targets includes the Eiffel Tower in Paris and the luxury Hotel Adlon near Berlin's Brandenburg Gate. Targets also include the Notre Dame Cathedral in Paris, as well as Berlin's Central Station and its landmark Alexanderplatz TV tower. One official said security has also been tightened around the British royal family.
The State Department alert is one step below a travel warning advising Americans not to visit Europe. Undersecretary of Management Patrick Kennedy said the administration is not advising that Americans avoid Europe despite the emerging intelligence.
"We are not, repeat not, advising Americans not to go to Europe," Kennedy said during a conference call. Kennedy said Americans should use "common sense" -- by quickly leaving the scene if they see unattended packages or hear "loud noises" and by minimizing the number of baggage tags that identify them as Americans on their luggage.
Kennedy said European and American officials have had "intensive discussions" for weeks about the threat, eventually leading up to the travel alert Sunday.
State Department officials said they could not recall another Europe-wide alert of this nature.
According to the department, it issued an alert after the 2004 train bombing in Madrid and updated its "worldwide caution" after the London bus attacks in 2005. The department also issued a "worldwide travel alert" last month urging U.S. citizens to be on the lookout for anti-U.S. demonstrations over the planned burning of Korans by a Florida church -- those plans were later called off.
The latest State Department alert urged U.S. citizens to "take every precaution" and adopt "appropriate safety measures" while traveling.
"Current information suggests that Al Qaeda and affiliated organizations continue to plan terrorist attacks. European governments have taken action to guard against a terrorist attack and some have spoken publicly about the heightened threat conditions," the State Department said.
"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. Terrorists have targeted and attacked subway and rail systems, as well as aviation and maritime services."
If the intelligence is correct, this would be the most involved role that bin Laden has played in plotting attacks since Sept. 11, 2001.
Japan also issued a travel alert for Europe on Monday, after 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."
The U.S. travel alert, which stressed that U.S. officials are working "closely" with the Europeans to track down terror threats, expires Jan. 31, 2011.
The alert urges Americans to register their travel plans online with the consular section of the U.S. Embassy at the State Department website. Travelers can also get updates on security conditions by calling 1-888-407-4747.
Fox News' Jennifer Griffin and Mike Emanuel and The Associated Press contributed to this report.