President Barack Obama has ordered a redoubling of counter-terrorism efforts in the face of a "credible but unconfirmed" threat ahead of the 10th anniversary of the September 11, 2001 attacks.

U.S. officials said the threat involved Washington D.C. and New York City -- the sites involved in the al Qaeda attacks a decade ago this Sunday that killed nearly 3,000 people.

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.

The White House said Obama was briefed on specific threat information on Thursday morning, and noted that the U.S. 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.

The U.S. 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."