The United States on Thursday raised its threat level for mass transit and train systems to code orange, or "high", after the London bombings, Homeland Security Secretary Michael Chertoff said.
"In light of today's attacks in London, the US government is raising the threat level from code yellow, or "elevated", to code orange, "high", targeted only to the mass-transit portion of the transportation sector," Chertoff said.
"This includes regional and inner-city passenger rail, subways and metropolitan bus systems.
"We're also asking for increased vigilance throughout the transportation sector," he said.
The threat level for the rest of the United States remains at yellow, the third point on the five-point US scale.
Chertoff said the move was a matter of "prudence" after the London attacks but was not in response to any intelligence suggesting strikes in the United States.
While declining to speculate on who planned the strikes, Chertoff said he had to consider the typical way that the al Qaeda terror group typically carried out simultaneous attacks, and "common sense" dictated a rise in the threat level.
"Currently, the United States has no specific credible information suggesting an imminent attack here," Chertoff said at a news conference, but he added: "Obviously we are concerned about the possibility of a copy-cat attack."
The announcement came after authorities in Washington, New York, Chicago and other US cities had hiked security on transport systems in response to the London bomb attacks targeting the bus and subway networks.
The nation-wide Amtrak railroad network also boosted security measures in response to the London attacks.
Chertoff, who several times recalled the March 2004 train bombings in Madrid, said he had no information that intelligence agencies had uncovered data which would lead them to suspect attacks like the ones in London were about to take place.
He said extra security measures on transportation networks would include an increased police presence, bomb detection equipment, more perimeter barriers and more frequent inspections of garbage cans and storage areas.