Washington, Sep 3 (IANS): The US has issued a worldwide travel alert for American nationals ahead of the 10th anniversary of the 9/11 terror attacks.

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

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

America remains at heightened vigilance against possible threats, Secretary of Homeland Security Janet Napolitano said Friday.

"As we approach the 10th anniversary of the September 11, 2001, terrorist attacks, the safety and security of the American public remains our highest priority," Napolitano said in a statement.

"While there is no specific or credible intelligence that Al Qaeda or its affiliates are plotting attacks in the US to coincide with the 10-year anniversary of 9/11, we remain at a heightened state of vigilance and security measures are in place to detect and prevent plots against the US should they emerge," she said.

Nearly 3,000 people were killed Sep 11, 2001 when Al Qaeda terrorists hijacked four passenger planes and crashed them into the twin towers of the World Trade Center in New York City, and the Pentagon in Arlington, Virginia. One plane crashed in Shanksville, Pennsylvania after passengers attempted to retake control of the aircraft.