U.S. issues worldwide 9/11 travel alert to Americans

The department said it had not identified any "specific threats" about possible attacks but that al-Qaida and its affiliates had "demonstrated the intent and capability to carry out attacks" against the U.S. and U.S. interests.

"In the past, terrorist organizations have on occasion planned their attacks to coincide with significant dates on the calendar," the State Department said.

The alert expires on January 2, 2012, it said.

Homeland Security Secretary Janet Napolitano said there was "no specific or credible intelligence that al-Qaida or its affiliates are plotting attacks" linked to the anniversary of the worst terror strikes on U.S. soil.

But she added: "We remain at a heightened state of vigilance, and security measures are in place to detect and prevent plots against the United States should they emerge."