The threat information was about three people who may have been dispatched to the U.S. to meet with associates to carry out the attacks, said a federal law enforcement official who was not authorized to speak publicly and asked not be identified.
President Obama was briefed about the matter Thursday morning and received updates during the day.
Department of Homeland Security spokesman Matt Chandler said documents recovered from the raid in May on Osama bin Laden's compound in Pakistan revealed al-Qaeda's interest to strike the U.S. on meaningful dates.
"In this instance, it's accurate that there is specific, credible but unconfirmed threat information," Chandler 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 said the threat, while credible, was "not corroborated." He urged New Yorkers to go about their business as usual.
"There is no reason to change any of your routines," he said at a news conference Thursday night. He also urged citizens to report anything suspicious.
New York Police Commissioner Ray Kelly said the department was extending officers' shifts and would be increasing police presence on the streets.
He also said officers would be conducting additional bag checks at subway stations and warned citizens they might encounter police checkpoints throughout the city.
The threat information did not prompt an immediate elevation of the national threat level.
Earlier Thursday, Homeland Security Secretary Janet Napolitano said there was "lots of chatter" among known terror associates and on jihadi websites in the run-up to Sunday's 10th anniversary of the attacks that killed nearly 3,000 people at the World Trade Center in New York, the Pentagon in Arlington, Va., and Shanksville, Pa.
In the past several days, federal, state and local authorities have dispatched thousands of personnel to sensitive locations in New York City, Washington and elsewhere to guard against possible disruptions.
Even without a confirmed threat, officials said the heightened state of alert was necessary because of al-Qaeda's stated intent to strike on the 9/11 anniversary.
Attorney General Eric Holder said there would be noticeable increases in law enforcement at national landmarks and public gatherings commemorating the anniversary.
Napolitano said more air marshals would be riding commercial airlines, which al-Qaeda hijackers converted into guided missles during the 2001 attacks.
Amtrak is stepping up screenings of passengers and baggage throughout its rail system, including the heavily traveled Northeast corridor.
Information seized from bin Laden's compound shortly after the terrorist leader was killed by Navy SEALs revealed that al-Qaeda considered the U.S. rail system as a possible target.
John O'Connor, Amtrak vice president and police chief, said the railroad is expanding patrols and bomb-detection teams.
White House chief counterterrorism adviser John Brennan said U.S. efforts in the aftermath of 9/11 — specifically the elimination of top-level terror leaders — have "made it much more difficult" for operatives to launch large-scale attacks.
Al-Qaeda "has taken it on the chin," Brennan said.
The death of bin Laden, the architect of 9/11, was the most significant and symbolic of the organization's losses.
Brennan said the material shows the al-Qaeda chief was "a little out of touch about how debilitated his organization was" after years of battle with U.S. and other forces.
"He was pushing for these major types of attacks," Brennan said. "I think his lieutenants were trying to tell him, 'We know what you want to do — great aspirations — but our ability to do that is degraded because we are losing people.'"