WASHINGTON — U.S. officials are investigating a “credible but unconfirmed” terrorist threat, with Washington and New York City being mentioned as possible targets, law enforcement and other officials said late Thursday.

President Obama was briefed on the threat Thursday morning and updated throughout the day as he prepared to address a joint session of Congress, a White House official said. Officials would not specify the nature of the information, but one federal official said it came from overseas.

“The United States government has already significantly enhanced its security posture in advance of the 9/11 anniversary to protect the country against possible terrorist threats,” said the White House official, who spoke on condition of anonymity because the threat information is not public. “Nevertheless, the president directed the counterterrorism community to redouble its efforts in response to this credible but unconfirmed information.”

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

Members of Congress were also briefed on the threat.

“There were very, very specific facts that were made known in this threat,” Rep. Peter King, R-N.Y., told CNN. “I would tell people right now to go about their lives. There’s no need to panic. We don’t know if this threat is real yet. It’s being tracked down.”

One federal law enforcement official, speaking on condition of anonymity, said: “Given the dates that are coming up, nobody wants to underplay anything. The government is going to do everything it can to run this to the ground and assess its accuracy.”

As the anniversary nears, security was already being ramped up nationwide, particularly in New York, where Obama and former President George W. Bush will mark the occasion at Ground Zero inside what police call a “frozen zone.”

Law enforcement officials have said they are acutely aware that before Osama bin Laden was killed, he seemed fixated on attacking the United States again on Sept. 11.

In the trove of digital and handwritten materials found at bin Laden’s compound in Pakistan in May, there were numerous references to the 10-year anniversary of the attacks on New York and Washington. The material also contained various inchoate ideas about how al-Qaida might construct a terrorist operation, according to law enforcement and intelligence officials.