Obama directs officials to redouble efforts amid 9/11 threat

"The President directed the counterterrorism community to redouble its efforts in response to this credible but unconfirmed information," a White House official said.

"The President was briefed on this specific threat information this morning and has been updated on it throughout the day," the official added.

"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," the official said.

In a statement, the Department of Homeland Security said that there is a 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. 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," it 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. We continue to ask the American people to remain vigilant as we head into the weekend," said the official.

New York Mayor Michael Bloomberg said the terror threat to the city ahead of the 9/11 anniversary is "credible but has not been corroborated".