Manila - Four people, including a three-year-old boy, were killed when pirates attacked a passenger vessel in the southern Philippines, police said Friday.

Eight people were wounded in the attack off the coast of Parang town on Jolo island, 1,000 kilometres south of Manila, according to Superintendent Danilo Bacas.

Bacas, a regional police spokesman, said the vessel was on its way to Laminusa Island when gunmen onboard a separate boat opened fire.

Investigators are still trying to determine the motive for the attack.

Piracy is rampant in southern Philippine seas, where criminals often demand protection money. Pirates frequently target cargo ships ferrying commodities between islands. (dpa)