US officials said Thursday evening they have "specific, credible but unconfirmed" information about a potential threat against the United States coinciding with the 10th anniversary of the September 11 terrorist attacks, CNN reported.

"We have received credible information very recently about a possible plot directed at the homeland that seems to be focused on New York and Washington, D.C.," a senior administration official told CNN Pentagon Correspondent Barbara Starr.

The official said 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," the official said. "Not enough is known about the potential operatives and their plotting."

The threat is believed to entail a vehicle bomb, but "we cannot rule out other means," a US official said.

A senior law enforcement official involved in briefings about the matter told CNN Justice Department Producer Terry Frieden that his best information is that the three individuals had not yet entered the United States.

The information came in around noon Wednesday, one official said.

A senior federal law enforcement official intimately involved in counterterrorism efforts told CNN Chief National Correspondent John King authorities "aren't sure if this is or real or just chatter."

The official confirmed the threat was vehicle-related.

New York City Mayor Michael Bloomberg told reporters Thursday night that while additional police will be deployed around the city, "There's no reason for any of the rest of us to change ... our daily routines."

Gov. Andrew Cuomo urged residents to be cautious, but said "there is no reason to panic."

In Washington, local officials said the city has a robust plan in place and has been on heightened alert since September 1.

"I want to urge all of our citizens ... to remain calm and let our law enforcement professionals do their job. At the same time, we ask people to remain vigilant," Mayor Vincent Gray said. "If you see something, say something."

Unattended vehicles around key infrastructure will be towed, Washington Metropolitan Police Chief Cathy Lanier said.

Federal officials said they were taking the threat seriously, while evidently trying to temper the news by saying such threats are commonplace during key events.

"It's accurate that there is specific, credible but unconfirmed threat information," said Matthew Chandler, spokesman for the Department of Homeland Security. "As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days. Sometimes this reporting is credible and warrants intense focus, other times it lacks credibility and is highly unlikely to be reflective of real plots under way.

"Regardless, we take all threat reporting seriously, and we have taken, and will continue to take, all steps necessary to mitigate any threats that arise. We continue to ask the American people to remain vigilant as we head into the weekend," Chandler said in a prepared statement.

A Department of Homeland Security official, speaking on background, said, "We will continue to respond appropriately to protect the American people from an evolving threat picture in the coming days and beyond. This may include an increased law enforcement presence at airports and other transit hubs, land and sea ports of entry, federal buildings, and other high-profile and critical infrastructure locations."

The information originated from the tribal border area of Pakistan and Afghanistan, a federal official told CNN producer Mike Ahlers.

Another source gave CNN conflicting information about possible details of the threat.