Heavily armed police were on alert in and around New York City Friday after US officials warned of a "credible" but unconfirmed bomb threat on the 10th anniversary of the 9/11 attacks, AFP reported.

"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. "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."