WASHINGTON (AP) -- The Bush administration raised the terror alert a notch to code orange for the nation's mass transit systems on Thursday, responding to a spate of deadly rush-hour bus and subway bombings in London.
''Obviously we're concerned about the possibility of a copycat attack,'' said Homeland Security Secretary Michael Chertoff.
The heightened alert will apply to ''regional and inter-city passenger rail, subways and metropolitan bus systems,'' Chertoff said at a news conference.
Chertoff said that U.S. authorities have ''no specific credible evidence'' pointing toward an attack in the United States. At the same time, he said, ''we are also asking for increased vigilance'' particularly in the U.S. transportation system.
He stressed that authorities are not asking Americans to avoid using their subways and bus systems in the light of the worst attack in London since World War II. To the contrary, he said those who use mass transit should continue to do so.
About 29 million people take commuter trains, subways and buses daily in the United States, with the New York City area accounting for about a third of the total, said Alan Pisarski, a Washington-based national transportation policy analyst. The next-largest systems are Chicago, Washington, Boston and Philadelphia. San Francisco has the largest system on the West Coast.
Chertoff told reporters he was not aware of any specific evidence that had foretold the attacks in London. Dozens of rush-hour commuters were killed and hundreds injured when four blasts went off in the city's subway system and a bus.
''I think our transit systems are safe,'' Chertoff said, adding that there have been vast improvements in the nearly four years since terrorists struck the United States on Sept. 11, 2001.
The terror alert has not been raised in the United States since last August, when the Homeland Security Department increased the threat level to orange, or high, for the financial sector in New York and New Jersey in the run up to the November elections.
President Bush, in Scotland for a meeting of the Group of Eight leaders, conferred in a secure video conference with national security and homeland security officials in Washington.
''I instructed them to be in touch with local and state officials about the facts of what took place here and in London,'' Bush told reporters from a summit of world leaders. Bush said he urged caution ''as our folks start heading to work.''
The State Department told all U.S. embassies to review their security arrangements. Secretary of State Condoleezza Rice telephoned British Foreign Secretary Jack Straw offering any assistance his government might require, said a State Department official, who spoke on condition of anonymity because the official was not authorized to be identified.
The U.S. Embassy in London is secure, but the consular section, which deals with visas and other routine business, is closed, the official said.
Rice has not changed plans to make a trip to Asia beginning Friday, with China the first stop, said Sean McCormack, the department spokesman. ''Her trip will proceed as scheduled,'' he said.
U.S. officials were trying to determine, meanwhile, whether an al-Qaida cell's claim of responsibility for the London attacks was credible. A U.S. official speaking on condition of anonymity because events were still unfolding said analysts were sifting through recent intelligence for evidence that other attacks might be in the works.
A senior U.S. counterterrorism official, speaking on condition of anonymity, said that because the attacks were well-coordinated and appeared fairly sophisticated, they were consistent with al-Qaida's methodology.
Recent intelligence indicated that London was considered a prime target for Islamic extremists, in part because al-Qaida was having difficulty getting people into the United States, the official said.