Eight fishermen including two boys have been shot and beheaded in a gruesome attack at sea off the troubled southern Philippines, police said Saturday.

The victims were among a group of fishermen who went missing after being attacked in the Moro Gulf off Mindanao island on Dec 26, said regional police spokesman Ariel Huesca.

Five headless corpses were first found aboard a boat drifting at sea on Friday by police who had gone in search of the missing fishermen, said Senior Inspector Joel Lozano, police chief of the district where the boat was taken.

"When we further inspected the boat (on Saturday morning), we discovered more headless bodies beneath the floorboard.... In all, it contained eight bodies," Lozano told reporters.

The corpses, including those of two boys aged about 12 and 14, were all decomposing, suggesting they were killed a few days earlier, Lozano said.

One fisherman remains missing after the attack by unknown gunmen, Huesca said in a written report.

The motive of the attack is not known.

The waters off the southern Philippines are rife with piracy, and the region is also a hotbed of Islamic militants.

The nine fishermen had gone missing a day after Christmas following the attack off the town of Olutanga, Huesca said.

Two other wounded fishermen in the party were recovered shortly after the attack and taken to hospital for treatment, he added.

The boat with the headless corpses was found adrift eight days later off the coastal village of Manicahan on Zamboanga's outskirts, about 80 kilometers southwest of Olutanga.

The 11 fishermen are all natives of Sangali, a fishing village adjacent to Manicahan, Huesca said.

© (c) 2014 AFP