U.S. citizens should "exercise extreme caution" when traveling to the Philippines, the U.S. State Department said today in a statement.

"Targeted sites may be public gathering places including, but not limited to, airports, shopping malls, conference centers, and other public venues," said the statement.

The warning specifies the island of Mindanao and the Sulu Archipelago as particularly dangerous and at great risk of harboring terrorist activity. Manila might also be included, the statement said.

Travelers should be aware of heightened police and military presence in these areas and of kidnap-for-ransom gangs that are active in the Philippines and target foreigners, the department said, adding that those who do visit the Philippines should register with the U.S. Embassy to be notified of emergencies.