The U.S. government on Friday issued a worldwide travel alert for American citizens as the tenth anniversary of 9/11 terrorist attacks is nearing.

The warning, issued by the U.S. State Department, said no specific threats had been identified, but terrorist groups had a tendency to launch attacks on significant dates in the past.

The State Department urged Americans overseas to take precautions, encouraging them to register to the State Department website to receive the latest security information.

U.S. Homeland Security Secretary Janet Napolitano said on Monday there was no credible intelligence indicating al-Qaida was planning attacks for the 9/11 anniversary, but the U.S. remained at "a heightened state of vigilance."