US on alert after \'credible\' 9/11 bomb threat

"There is specific, credible but unconfirmed threat information," the Department of Homeland Security said Thursday, as the White House confirmed President Barack Obama had ordered boosted counterterrorism efforts.

"We have taken, and will continue to take all steps necessary to mitigate any threats that arise," the department added in a statement.

A White House official confirmed that President Barack Obama had "directed the counterterrorism community to redouble its efforts in response to this credible but unconfirmed information."

Federal officials in Washington, speaking on condition of anonymity, said there was a potential terrorism threat involving bomb-laden vehicles against either the capital or New York.

Few details were given, but one US official told AFP a car bomb was "at the top of what we would be looking for."

"There\'s enough information that\'s specific and credible that you have to run it to ground," the official said, adding: "I would stress that this is unconfirmed."

Although there was no immediate change to the official national threat level, New York authorities immediately announced sweeping extra measures, including vehicle checkpoints.

The police department "is deploying additional resources... some of which you will notice and some of which you will not," Mayor Michael Bloomberg told reporters.

City police commissioner Raymond Kelly told the press conference that measures included police "trained in heavy weapons positioned outside of Manhattan to respond citywide."

Extra shift hours would effectively increase by a third the size of patrols around New York, with checks on ferries, tunnels, bridges and landmarks, Kelly said.

There will be increased towing of illegally parked cars and more bomb detection sweeps in car parks, as well as "increasing the number of bag inspections on the subway," Kelly said.

Bloomberg said the threat was credible, but "at this moment has not been corroborated. I want to stress that."

US Senator Susan Collins, a ranking member of the homeland security committee, confirmed that she had received a classified briefing on a "specific and credible" terrorist threat Thursday morning.

"I am confident that the administration is taking the threat seriously and sharing intelligence with state and local enforcement officials in the targeted locations," she said in a statement.

Earlier, US military bases had raised their alert levels, but officials would not say whether this was related to the new threat report.

The scare came days ahead of Sunday\'s anniversary ceremonies for the September 11, 2001, attacks, when Obama and his predecessor George W. Bush are due in the city, along with large crowds.

Despite frequent threats and a string of failed plots, Al-Qaeda has not succeeded in mounting a major attack on US soil since 2001, when it hijacked passenger planes and crashed them into New York\'s World Trade Center, the Pentagon and a Pennsylvania field, killing nearly 3,000 people.

However, officials have warned of a possible backlash following the US commando operation in May that killed Al-Qaeda founder Osama bin Laden in a secret compound in Pakistan.

According to US officials, documents and computer files were seized at the compound showing bin Laden was considering strikes to coincide with the 9/11 anniversary.

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