AP Photo/Lauren Victoria Burke

Washington Metropolitan Transit Police, visibly carrying automatic weapons, increase their presence on the Metro in the nation's capital Thursday, July 7, 2005, following a series of explosions on the London transit systems.

AP Photo/Coke Whitworth

A railroad police officer patrols 30th Street Station in Philadelphia with an explosives-sniffing dog following a DHS request to increase security at public transportation facilities.

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.