Authorities in New York and Washington have stepped up security after learning that al Qaeda may be planning to detonate a car bomb in those cities, as the 9-11 anniversary nears.

Police in New York focused on boosting security around bridges and tunnels, establishing checkpoints and doing bomb sweeps of parking garages. Commuters were told to expect to see extra police on hand at major transit hubs, including Grand Central Terminal and Pennsylvania Station.

A spokesperson for the Port Authority of New York and New Jersey said there would be an increased police presence at all of its facilities.

In Washington, Police Chief Cathy Lanier said officers would be working 12-hour shifts for the immediate future and she warned city residents that unattended cars parked in unusual locations could be towed away.

Speaking in New York on Friday, Secretary of State Hillary Clinton said officials made the new terrorist threat public to ensure that Americans remained vigilant, and to set in motion a "great network of unity and support" aimed at protecting people from a potential attack.

"It is a continuing reminder of the stakes in our struggle against violent extremism," Clinton said.

The U.S. has been on high alert for the past two days, after it was revealed that authorities were in the midst of hunting down information about the purported al Qaeda plot. The threat has not been corroborated, but U.S. officials say they learned of its existence from a credible source.

"There's no certitude, we don't have the smoking gun, but we do have talk about using a car bomb," U.S. Vice-President Joe Biden told ABC's Good Morning America earlier on Friday.

There is also concern that three people may be trying to enter the U.S. to carry out the plot. At least one is believed to be an American citizen. Counterterrorism officials are also investigating names associated with the purported plot, but whether or not those names are fake remains unclear.

"We have been told that that was an intention to get people into the United States to do that," Biden said. "But we do not have confirmation."

Word of the apparent threat in the U.S. comes just days before the 10th anniversary of the Sept. 11, 2001 terrorist attacks, which killed nearly 3,000 people.

Concerns about possible attacks timed to coincide with the upcoming anniversary were raised after U.S. forces killed Osama bin Laden in Pakistan. During the raid they uncovered material suggesting that al Qaeda had considered attacking the U.S. on Sept. 11, 2011 and other important American dates.

Canadian officials have been briefed on the details of a developing threat, though there is no indication the danger extends north of the border.

CTV's Ottawa Bureau Chief Robert Fife said Canadian officials have spoken with their U.S. counterparts about the terror threat and are co-operating in any way that they can.

But senior government officials in Ottawa say they have no reason to believe that the threat extends north of the border.

"They say there is absolutely no threat aimed at Canada, they see nothing on the radar that would pose a threat to any Canadian city or any part of Canada at all," Fife told CTV News Channel on Friday morning.

With files from The Associated Press