Members of the public react as they walk past a New York Police Department Hercules team on patrol near Penn Station in New York August 24, 2011. [Agencies]

WASHINGTON - US President Barack Obama on Thursday ordered US counterterrorism forces to "redouble" their efforts to counter a possible terrorism threat surrounding the 10th anniversary of the September 11 attacks.

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