The United States and Britain have warned their citizens to stay off a southern Philippine region where suspected Muslim extremists are behind a wave of kidnappings, including those of 18 Indonesian and Malaysian tugboat crewmen.

The U.S. State Department warned Americans in an advisory Thursday to defer non-essential travel to the Sulu archipelago "due to the high threat of kidnapping of international travelers, increased threat of maritime kidnappings against small boats ... and violence linked to insurgency and terrorism there."

The British government on Wednesday issued a similar travel warning.

Abu Sayyaf militants have been suspected of seizing 18 Indonesian and Malaysian crewmen in separate attacks on three slow-moving tugboats, sparking a security alarm in the volatile region.