WASHINGTON (AP) - The Homeland Security Department asked authorities in major cities Thursday for heightened vigilance of transportation systems after a series of explosions on London transit systems.
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 in London," Bush told reporters from a summit of world leaders here. Bush said he urged caution "as our folks start heading to work."
In the nation's capital and several major cities, local authorities heightened security in transit systems. But Homeland Security Department spokesman Brian Roehrkasse said there were no immediate plans to raise the nation's threat level.
"We do not have any intelligence indicating this type of attack is planned in the United States," Roehrkasse said.
The Homeland Security Department asked authorities in major cities for "continued vigilance" of their transportation systems, Roehrkasse said.
U.S. officials were trying to determine, meanwhile, whether an al-Qaida cell's claim of responsibility for the attacks was credible. A U.S. official speaking on condition of anonymity because the situation was still unfolding said analysts were sifting through recent intelligence for any evidence of follow-up attacks here or abroad.
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.
Security was stepped up in Washington, with bomb-sniffing dogs and armed police officers patrolling subways and buses - ridden by about 1.2 million people a day - and looking for anything suspicious. Police carrying rifles rode some trains, and passengers were being urged to report any suspicious activity.
At the Pentagon, police officers said they had been placed on a heightened alert shortly after the London attacks.
Security around the massive home of the Defense Department, hit in the Sept. 11 attacks, was more visible than normal Thursday, with police officers in squad cars, on bicycles and on foot deployed in greater numbers than usual on the perimeter roadways and the Metro station.
Amtrak increased the number of police patrols and bomb-sniffing dog teams, and reminded railroad employees to be alert for suspicious activity, according to a company statement.
"These security measures are just taken as a precaution," said spokeswoman Tracy Connell said. "We will continue the heightened security threat level until we have a better understanding of what happened in London."
Several major cities heightened security on transit systems.
In Boston, Massachusetts Gov. Mitt Romney raised the security level on the city's transit system. Romney spokesman Eric Fehrnstrom said the heightened security level is similar to the "Code Orange" used by federal authorities.
Increased police coverage of subways, buses and transit stations was also ordered throughout New York City.
In Los Angeles, a police official said police had activated a special command center and officials were meeting to decide whether to upgrade security levels around the city.
A spokeswoman for the Chicago Transit Authority said transit officials were working with Chicago Police on additional security measures. CTA spokeswoman Kimberly Myles said announcements were being made to riders to be aware of their surroundings and to alert transit workers about suspicious packages.
Similar announcements were made in Cleveland and several other cities.