Heavily armed police were on alert in and around New York City Friday after US officials warned of a "credible" but unconfirmed bomb threat around the 10th anniversary of the 9/11 attacks.
"There is specific, credible but unconfirmed threat information," the Department of Homeland Security said Thursday, as President Barack Obama 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 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."
ABC News cited intelligence officials as saying the potential plot involved three individuals who entered the country by air last month with the intention of carrying out a vehicle-borne attack on or around the 9/11 anniversary.
It said the plot was hatched by Al-Qaeda's leader Ayman al-Zawahiri, who has vowed to avenge the killing of the group's founder Osama bin Laden in a US commando raid in Pakistan earlier this year.
Although there was no immediate change to the official US 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.
US Representative Peter King, also briefed on the intelligence, declined to discuss specifics but told CNN the threat included "very, very specific facts."
"There's no need to panic. We don't know if this threat is real yet. It's being tracked down," he added.
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, US officials have warned of a possible backlash following the killing of bin Laden in a secret compound in Pakistan in May.
US officials have said documents and computer files were seized at the compound showing bin Laden was considering strikes to coincide with the 9/11 anniversary.