WASHINGTON - The government is urging Americans to be vigilant about suspicious activity after British police defused a bomb in downtown London, but officials said they saw no potential terrorist threat in the United States ahead of next week's Fourth of July holiday.

The Department of Homeland Security said in a statement that it was in close contact with state and local authorities around the nation but had "no specific credible information suggesting a threat to the homeland at this time."

"At this time we are characterizing this as a localized incident in London," DHS spokeswoman Laura Keehner said.

"We encourage the public to enjoy the upcoming holiday but ask, as always, that they be vigilant and report suspicious activity to authorities," she said.

President Bush was briefed on the incident by national security adviser Stephen Hadley in Maine, where the president is at his family's home to meet Sunday and Monday with Russian President Vladimir Putin.

"We commend the British security services and local officials for their action today. U.S. officials are in contact with their U.K. counterparts and will continue to monitor the situation," said Gordon Johndroe, a spokesman for the National Security Council.

The 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.

U.S. counterterrorism officials, who spoke on condition of anonymity because the investigation is ongoing, said it was too early to say whether the foiled attack in London was strictly a British threat, or whether there could be related threats in the U.S. and elsewhere.

"At this point, you can't draw conclusions about what it may or may not mean about plotting against the U.S.," one official said. "Concern about that kind of plotting is always there."

The London threat comes at the same time as U.S. counterterrorism authorities are worried about terrorist activity in the tribal areas of Pakistan, where al-Qaida continues to have a presence, although officials said it was too early to know if there were links to extremist elements there.

A threat from that region would not be new to London, officials said. On July 7, 2005, four bombs exploded in the city's transit system, and investigators found that attack had links leading back to Pakistan.

However, a second U.S. counterterrorism official said it was also too early to know who may have been behind the foiled attack or if there were any links to extremist elements in Pakistan.

In London, the U.S. Embassy said in a statement to Americans that the "incident serves as a reminder to all Americans living and traveling abroad to remain aware of their surroundings at all times and be vigilant to suspicious activity."

At the Pentagon, the matter was brought up at a regular early morning briefing among members of the Joint Chiefs of Staff to make officials in Washington aware of the incident. A staff member said later that if there had been any request for Defense Department involvement in the investigation, he was not aware of it.

Earlier Friday, London police defused an explosive car loaded with gas cylinders, nails and a detonator after an ambulance crew reported seeing smoke coming from the vehicle in The Haymarket, near Piccadilly Circus, after an ambulance crew — responding to a call just before 1:30 a.m.

The area — packed with restaurants, bars, a cinema complex and theaters — was busy and buzzing at that hour. Haymarket links Piccadilly Circus to the north to the Pall Mall at its southern end.

A British security official, speaking on condition of anonymity because of the sensitivity of the information, told The Associated Press that Britain's domestic spy agency MI5 also would examine possible connections between the bomb attempt and at least two 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.