Heightened operations

The attack came two days after security forces raided an Abu Sayyaf base in nearby Indanan town.

Two marines and eight Abu Sayyaf fighters were killed in that confrontation, including two sons of one Abu Sayyaf commander, and the son of another.

The Philippines has stepped up its offensive against Abu Sayyaf to stop the group's kidnap-for-ransom activities in the south after the US raised concerns over such threats in the region.

The Abu Sayyaf is holding Eugenio Vagni, an International Committee of the Red Cross (ICRC) engineer, who will enter his sixth month in captivity on Monday.

Vagni was kidnapped along with Andreas Notter, a Swiss national, and Mary Jean Lacaba, a Filipina, by members of the group after they inspected a sanitation project at a local prison on Jolo island on January 15.

Notter walked to freedom on April 18 and Lacaba was freed on April 2. A ransom was suspected to have been paid for the two ICRC workers.