Washington: US counter-terrorism officials are assessing "specific, credible but unconfirmed" information about a threat of an attack in New York City or Washington using a car or truck bomb coinciding with the 10th anniversary of the September 11 terrorist attacks.

The plot was believed to involve three individuals, including a US citizen. US officials believed the threat was a vehicle laden with explosives, but "the intelligence picture is not completely formed," reported a news channel citing a senior administration official. "Not enough is known about the potential operatives and their plotting."

The Department of Homeland Security spokesman Matthew Chandler confirming the reports said: "It's accurate that there is specific, credible but unconfirmed threat information."

The information originated from the tribal border area of Pakistan and Afghanistan, a federal official the news channel.

Peter King, Republican chairman of the House Homeland Security Committee, said very specific details were made known to lawmakers about the threat. "Many agencies are looking at this from every possible angle," he said.

"The President was briefed," a senior administration official said. "He directed that the intelligence community and the administration take all necessary steps to ensure vigilance."

Earlier on Thursday, Homeland Security Secretary Janet Napolitano told reporters that intelligence officials had picked up "lots of chatter" on jihadi websites and elsewhere about the impending 10th anniversary of the September 11 terrorist attacks "and we're taking it all seriously," but nothing yet that warranted issuing a threat advisory.