But the officials used strong caveats when discussing the threat information privately, with a national security official cautioning that experts thought the threat would ultimately not check out.
New York City Mayor Michael Bloomberg also stressed that the threat had not been corroborated, even as he announced heightened security measures "some of which you may notice, some of which you may not notice."
"There is no reason for any of the rest of us to change anything in our daily routines," he told a news conference.
Still, Bloomberg asked citizens to report suspicious or dangerous activity, adding: "Over the next three days we should all keep our eyes wide open."
The White House said Obama was briefed on specific threat information on Thursday morning, and noted that the government had already "enhanced its security posture" ahead of the anniversary.
"Nevertheless, the President directed the counterterrorism community to redouble its efforts in response to this credible but unconfirmed information," a White House official said, speaking on condition of anonymity.
Documents discovered in Osama bin Laden's compound in Abbottabad, Pakistan, after he was killed in a raid in May by Navy SEALs, highlighted his persistent interest in attacking the United States around the anniversary of the 2001 attacks. But it is unclear if the plans ever evolved beyond aspiration.
Bloomberg said he spoke with the head of New York's public transportation authority, which was hiking security. He added: "For the record, I plan to take the subway tomorrow morning."
New York City Police Commissioner Ray Kelly announced measures including more bag inspections on the subway, more bomb-sniffing dogs on patrol and increased deployment of radiation monitoring equipment.
"There will be increased focus on tunnels and bridges and infrastructure in general, as well as landmark locations, houses of worship and government buildings," he said.
The Department of Homeland Security, which said only last week that there was no credible information that al Qaeda was plotting an attack around the September 11 anniversary, declined to offer details on the threat.
It cautioned that there were always threat reports before important dates like the anniversary of the September 11 attacks.
"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," spokesman Matt Chandler said.
"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."
A second law-enforcement source played down an ABC News report about missing rental trucks -- saying the vehicles had been recovered and there was no connection to terrorism.