Another tugboat was hit by pirates on Saturday night in the Celebes Sea between the Philippines and Indonesia. Three Indonesian crew were taken hostage when five armed men boarded the tug.
The pirates are believed to belong to Abu Sayyaf, the southern Philippine Islamist group which has been responsible for a number of kidnappings in the area this year.
Abu Sayyaf now has more than 20 seafarers held hostage.
The navies of Indonesia, Malaysia and the Philippines have vowed to up patrols in the Celebes and Sulu seas in a bid to fight Abu Sayyaf.