Five people were killed and 17 injured today when a bomb exploded at a small retail store in the southern Philippines, the army said.
A spokesman said no group had claimed responsibility for the attack on the island of Jolo, but local police suspected that al-Qaeda-linked militant group Abu Sayyaf was behind it.
Brigadier-General Alexander Aleo, the army's island commander, said Abu Sayyaf, a small but deadly local Muslim rebel group, was behind the attack on a shop in the heart of the main market in Jolo.
"This is definitely the handiwork of the Abu Sayyaf," said Aleo. "Based on our initial investigation, a cellphone was used as a triggering device to a bomb made out of ammonium nitrate."
"We all knew that the Abu Sayyaf got training in making bombs from Indonesian militants from Jemaah Islamiah. This could be a product of that technology transfer."
The provincial police chief said one man had been taken in for questioning, but he declined to say whether he was a suspect or a witness to the worst bomb attack so far this year on the mainly Muslim island.
Troops in the area had gone on alert to prevent an escalation of violence, the army said.
Last month, a crude bomb ripped through a row of night clubs outside an army base on Jolo, killing a man and wounding 13 people, days before 250 US troops were due to conduct humanitarian missions on the island.
Jolo is the stronghold of Abu Sayyaf. The group is blamed for the worst terror attack in the Philippines - the bombing of a ferry in February 2004 that killed more than 100 people.
Abu Sayyaf is believed to get training and funding from al-Qaeda and Jemaah Islamiah, the Southeast Asian militant network.
On Washington's blacklist of terrorist groups, Abu Sayyaf has also kidnapped and beheaded foreign and Filipino tourists and church workers.