The threat level for a terror attack in Canada has not increased following information of a possible plot of a car bombing in Washington or New York on the anniversary of Sept. 11, 2001 the RCMP says.

"The RCMP has no information at this time that indicates that Canadians are more at risk than usual,"  RCMP Sgt. Julie Gagnon told CBC News.

Counterterrorism officials in the U.S. have been chasing a credible but unconfirmed tip that al-Qaeda has plans to set off a car bomb in New York City or Washington, with bridges or tunnels as potential targets. It was the first word of a possible "active plot" timed to coincide with commemoration of the group's attacks in the United States a decade ago.

Public Safety Minister Vic Toews backed the RCMP assessment.

"In respect of Canada, I can't point to any specific threat that might occur during this weekend but I think that all of our agencies are on full alert on a weekend like this," Toews told Evan Solomon on CBC News Network's Power and Politics.

"We've certainly been in touch with [U.S. Department of] Homeland Security.… I haven't talked to [Homeland Security Secretary] Janet Napolitano directly and at this point I don't see a need to do that but I can assure you that our agencies, because of the increased integration ... are in fact working very close together ... A threat against the United States is a threat against Canada."   Nevertheless, Toews added, his department "is aware of the potential threats that may occur during this weekend and we have responded appropriately. And that does involve the utilization of resources."

Police in New York and Washington increased their already stepped-up staffing levels. In New York City, authorities were stopping vehicles at the 59th Street bridge on Friday, causing a major traffic backup. National Guard troops and transit police carrying assault rifles watched the crowds at Penn Station.

White House officials said U.S. President Barack Obama had no plans to change his scheduled trips to New York's Ground Zero, the Pentagon and Shanksville, Pa., sites of the 2001 attacks, on Sunday to mark the anniversary.

This latest threat "should not surprise any of us," Secretary of State Hillary Rodham Clinton said during a speech in New York.

Telling the public "is intended to enlist the millions and millions of New Yorkers and Americans to be the eyes and the ears of vigilance," she said.

Pursuing 3 people

Law enforcement officials were pursuing three people who may be travelling to the U.S. or who have recently entered the country, based on the detailed information received by the U.S. intelligence community late Wednesday, officials said.

Senior U.S. counterterror officials said Friday that authorities have a general sense of who is behind the plot and who may be tasked to carry it out, but they do not have exact identities yet. They said there is at least one U.S. citizen among the three people.

The intelligence suggested that al-Qaeda planned to car bomb one of the two cities that were hit 10 years ago, in co-ordinated attacks that killed nearly 3,000 people.

Vice-President Joe Biden said Friday that there was no confirmation that anyone had travelled into the U.S. for such a plot although the tip came from a credible source. "There's no certitude," he said.

Worried about 'lone actor'

"The thing we are all most worried about is what they call a 'lone ranger,' a lone actor, not some extremely complicated plan like it took to take down the World Trade [Centre] towers," said Biden, who appeared on the trio of network morning TV shows Friday.

U.S. officials said the source of the terror tip indicated that al-Qaeda's new leader, Ayman al-Zawahri, was involved in planning the plot. But the official also said that many in the intelligence community question that and other aspects of the source's information.

The nation's terror alert level has not changed, although raising it was under consideration Thursday night.

At the Pentagon, officials said there have been no changes to military base security levels since they were upgraded early in the week, before the threat information came in.

Officials said that so far they have no reason to believe that there is a direct connection between this new threat and the information found in the compound laying out al-Qaeda's aspirational goals.

The threat came in a single piece of information and was so specific, and came at such a time of already heightened alert, that it could not be ignored, officials said.

"There is specific, credible but unconfirmed threat information," the head of the FBI's New York division, Janice Fedarcyk, said. "As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days."

New York Mayor Michael Bloomberg told reporters that police there were deploying additional resources around the city but that New Yorkers should go about their business as usual, and the city's observance of the attacks will go on as planned.

In Washington, law enforcement officials said they were working 12-hour days indefinitely, and police Chief Cathy Lanier said unattended cars parked in unusual locations risked being towed.

With files from CBC