Hours earlier, a solider was wounded in a gun battle with suspected rebels near Zamboanga city, about 400 kilometres (250 miles) west of General Santos.

The clashes came days after 15 people were killed and scores more were wounded in a series of bomb attacks in the city, blamed on the rebel movement.

Arsenal of weapons

Regional police chief Bartolome Baluyot said up to 15 men in two speed boats opened fire on the police patrol after being ordered to stop.

"When they were about 25 metres (82 feet) away, we fired warning shots.

"Instead of signalling that they were not the enemy, they rained gunfire and M-203 grenades on us. So I ordered, 'fire'," he said.

Senior Superintendent Baluyot said four of the attackers were shot dead by police and the others escaped.

The Abu Sayyaf is thought to be behind a recent bomb attack

He said officers recovered two rifles, a grenade launcher, two handguns, a telescope, and two extortion letters intended for local businesses.

The letters were signed by Abu Muslim al-Ghazi, who earlier claimed responsibility on behalf of the Abu Sayyaf for the triple bombing in the city on 21 April.

The movement has carried out a string of kidnappings and has been linked to Osama bin Laden's al-Qaeda terror network.

More than 1,000 US troops are training Philippine forces in the fight against Abu Sayyaf on Basilan island, 350 kilometres (215 miles) west of General Santos.

The rebels say they are fighting for an independent Islamic state in the south of the predominantly Christian country.