WASHINGTON (CNN) -- The U.S. government is not aware of any specific threat of a terrorist attack to coincide with the 10th anniversary of the September 11, 2001 attacks, Homeland Security Secretary Janet Napolitano said in a statement Friday.
But overseas travelers should remain wary, the State Department said in a worldwide travel alert.
"In the past, terrorist organizations have on occasion planned their attacks to coincide with significant dates on the calendar."
Ceremonies observing the anniversary of the attacks are scheduled for New York, Washington and many other communities around the country. Napolitano reminded state and local authorities and individuals to remain watchful and to report any suspicious activities.
"While there is no specific or credible intelligence that al Qaeda or its affiliates are plotting attacks in the United States to coincide with the ten 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 United States should they emerge," she said.
The State Department travel alert adds to a July worldwide caution alerting travelers to the threats posed by the May killing of Osama bin Laden by U.S. forces and the ongoing instability in several countries in the Middle East and North Africa.
Such alerts are issued when short-term conditions "pose imminent risk to the security of U.S. citizens," according to a State Department explanation.
"While we have not identified any specific threats from (al Qaeda) affiliates and allies to attack the United States or our interests on the 9/11 anniversary, U.S. citizens should be aware that (al Qaeda) affiliates and allies have demonstrated the intent and capability to carry out attacks against the United States and our interests around the world," the State Department said in its alert.
U.S. defense officials also will be "keenly aware, watching closely" for possible threats against military installations, said Pentagon spokesman Col. Dave Lapan.
"After the raid on the bin Laden compound we turned up information that suggested that attacks around the anniversary of 9/11 are something they were pursuing but again that is very general," he said. "There is nothing specific or credible at this point."