A heavily armed member of the New York Police Department patrols Times Square Friday in New York. New York strengthened its already-tight security as a precaution after a bomb threat in London, putting additional police in Times Square and the mass transit system, officials said.

Washington ? Unexploded car bombs in London led to extra patrols in the United States on Friday, but Bush administration officials said they saw no special terrorist threat heading toward next week’s Fourth of July holiday.

Homeland Security Secretary Michael Chertoff urged Americans to be vigilant but said there were no immediate plans to raise the U.S. national threat level, now at yellow, or elevated.

Said White House press secretary Tony Snow: “There is no specific or credible evidence of any threat of any kind against the United States of America.”

Snow said British authorities had not yet been able to determine if there were a link to any terrorist group.

“Look, it’s terrorism, but we don’t know if there – there’s no definite, there’s no established connection with any organization at this point,” he said. Snow said U.S. officials “remain very aggressively engaged” with the British.

Bush was briefed in Kennebunkport, Maine, about the London investigation. At the White House, homeland security adviser Frances Fragos Townsend called a meeting of top officials, inviting Vice President Dick Cheney, Secretary of State Condoleezza Rice, Defense Secretary Robert Gates, Chertoff, Treasury Secretary Henry Paulson and Attorney General Alberto Gonzales, among others.

Throughout the country, local law enforcement agencies received an FBI bulletin saying that while July 4 festivities might make an attractive target for terrorists, authorities had no credible evidence that such an attack was planned, according to someone who read the report and summarized it for The Associated Press.

Without intelligence linking the London vehicles to any group or conspiracy, investigators there were poring over the cars looking for fingerprints or other clues.

In New York City, Mayor Michael Bloomberg said police would work extra hours in more locations.

“We’re going to ramp up a little bit, but nothing dramatic,” Bloomberg said on his weekly radio show.

The extra police presence was ordered out of a sense of precaution, not panic, officials said.

“We want to just be on the safe side here and employ additional resources over the weekend,” Police Commissioner Raymond Kelly said.

The police department increased patrols at high-profile tourist areas such as Times Square, as well as the subways. Officers were told to give extra attention to parking garages and any suspicious vehicles.