Three days before the 9-11 anniversary, American officials say they're investigating "credible but unconfirmed" information that al Qaeda may be plotting to bomb bridges or tunnels in Washington or New York.

News of the threat comes amid heightened security in both cities, days ahead of the 10-year anniversary of the Sept. 11, 2001 attacks.

New York Mayor Michael Bloomberg held a news conference Thursday night, saying that the threat information had not been independently corroborated.

But the city's police force is "deploying additional resources," he said, suggesting that more uniformed and plain clothes officers will be patrolling the streets.

"Over the next few days we should all keep our eyes wide open," Bloomberg advised New Yorkers. "If you see something, say something."

Security measures are also being stepped up at local bridges and tunnels, the mayor said.

"Just for the record, I plan to take the subway tomorrow morning and feel just as safe as when I took it this morning," he added.

Ed Turzanski, a senior fellow with the Foreign Policy Research Institute and former CIA agent, told CTV News Channel that security personnel are on alert for three Pakistani nationals who recently entered the U.S. and a fuel tanker that may be missing.

However, the threat level has not been raised because the information hasn't been corroborated, he said.

U.S. President Barack Obama was made aware of the threat Thursday morning and instructed counterterrorism authorities to respond accordingly, a White House official said.

"It's accurate that there is specific, credible but unconfirmed threat information," Homeland Security Department spokesman Matt Chandler said in a statement. "As we always do before important dates like the anniversary of 9-11, we will undoubtedly get more reporting in the coming days."

Information obtained in the raid on Osama bin Laden's compound in May suggested that al Qaeda may have been interested in attacking the United States on the 10th anniversary of 9-11 or on other significant dates.

On Thursday morning, Homeland Security Secretary Janet Napolitano said there was "a lot of chatter" about the 9-11 anniversary, but no information about a specific threat.

With files from The Associated Press