"There is specific, credible but unconfirmed threat information," the Department of Homeland Security said, 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.
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's authorities immediately announced sweeping extra measures.
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 carparks, as well as "increasing the number of bag inspections on the subway," Kelly said.
In addition, "the public is likely to see and may be somewhat inconvenienced by vehicle checkpoints at various locations."
Bloomberg said the threat was credible, but "at this moment has not been corroborated. I want to stress that."
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 President Barack Obama and his predecessor George W. Bush are due in the city, along with large crowds.
On 9/11, two hijacked planes hit New York's World Trade Center, collapsing the iconic twin towers, a third plane plowed into the Pentagon and a fourth crashed into a Pennsylvania field when passengers overcame the hijackers.
Despite frequent threats and a string of failed plots, al Qaeda did not succeed in mounting another major attack on US soil.
However, officials have warned of a possible backlash following al Qaeda founder Osama bin Laden's killing by US forces in May.
US Navy commandos tracked down and killed Bin Laden at his hideout in Abbottabad.
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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