WASHINGTON -- Unexploded car bombs in London led to extra patrols in the United States 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. He was in Kennebunkport, Maine, where President Bush will meet Sunday with Russian President Vladimir Putin.

Snow said British authorities had not yet been able to determine if there was 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.

A police officer cordoned off the Piccadilly Circus area of central London on Friday. British police defused a bomb found in a car in central London on Friday, and the new government called an emergency meeting of senior security chiefs to investigate the situation. (HORIZ AKIRA SUEMORI —Associated Press)

Bush was briefed in Kennebunkport 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.

A heavily armed member of the New York Police Department patroled Times Square on 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. (NY MARY ALTAFFER ~ Associated Press)

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.

A British security official said that Britain's domestic spy agency MI5 was examining whether there was any connection in London with similar foiled plots -- including a planned attack on a West End nightclub in 2004 and a thwarted attempt to use limousines packed with gas canisters to attack targets in London and New York.

Kelly said the similarity between the first bomb found in London Thursday night and the method of the limousine plot was partly responsible for New York's heightened security.

The London car bomb near Piccadilly Circus was powerful enough that it could have caused "significant injury or loss of life" -- possibly killing hundreds, British anti-terror police chief Peter Clarke said.