MANILA, Jan. 10 (Xinhua) -- Five gunmen attacked a fishing boat on Tuesday off Zamboanga City in the southern Philippines, killing at least eight crew members on board.

Army spokesman Filemon Tan said in a statement the boat with 15 crew on board was struck by shells fired by the gunmen aboard a speed boat around 7:30 a.m. off Laud Siromon in Dita village in Zamboanga City.

"Eight crew were killed in the straffing," Tan said, adding the five other crew jumped off the boat and survived the attack by swimming to a nearby island.

He added two others who remained on the boat were unharmed.

Tan said "personal grudge" or "extortion" could be the motives for the attack.

He said police and military personnel were sent to the area to look into the incident.