A grenade has exploded near a crowded market in the Philippines, killing at least five people, including a 14-year-old boy.
The explosion took place in Jolo town, on Jolo island. It comes days after the military launched air and ground assaults against the Muslim extremist group Abu Sayyaf.
The military operation follows recent rebel attacks that have killed at least 11 soldiers on Jolo.
The explosion occurred as US troops began joint military exercises with the Philippine army on the nearby island of Basilan, 900 km (560 miles) south of Manila.
"Some of them were screaming for help," army Colonel Romeo Tolentino told reporters, referring to victims of the sidewalk market blast in central Jolo town.
"We are looking at two angles. It could be an act of retaliation by gangs operating extortion activities at public markets. The other possibility is it could be a diversionary attack of the Abu Sayyaf," he said, referring to a southern Philippine Islamic group linked to Osama bin Laden's al-Qaeda network.
Colonel Tolentino said they police and military reinforcements has been sent to the area. He said no suspects had been identified so far.
The islands of Jolo and Basilan are lairs of the Abu Sayyaf, which has been holding an American missionary couple and a Filipina nurse on Basilan for more than eight months.
The US troops are on a six-month training mission in the Philippines in what is the biggest expansion of Washington's war on terror outside of Afghanistan.
PA