MANILA, Philippines – The Philippine military said Abu Sayyaf militants are suspected of kidnapping four Filipino fishermen in the latest such attack by the ransom-seeking gunmen despite increased security in the southern region.

Other fishermen found the F/B Ramona 2 fishing boat Tuesday in the Celebes Sea but its skipper, mechanic and two crewmen and the boat’s radio and GPS equipment were missing, regional military spokesman Maj. Filemon Tan said.

Tan said troops were alerted to look for the missing fishermen who may have been taken to nearby Sulu province, where the militants are holding other hostages in jungle encampments.

Kidnappings of boat and cargo crewmen by the Abu Sayyaf and allied gunmen have continued despite efforts from the Philippines, Malaysia and Indonesia to bolster sea security.

Without any known foreign support, the Abu Sayyaf has depended largely on kidnappings for ransom, extortion and acts of banditry to survive. The brutal group has been blacklisted by the United States and the Philippines as a terrorist organization for bombings, kidnappings and beheadings.