Zamboanga City - At least nine people were killed Monday in a powerful bomb blast that ripped through a grocery store on a southern Philippine island, police said.

More than 20 other people were injured in the explosion, which occurred shortly after the lunch break at the Jolo Cooperative Consumer store on Jolo island, 1,000 kilometres south of Manila.

Senior Superintendent Ahirom Ajirin, provincial police commander, said one man suspected of being involved in the bombing in Jolo city was taken into custody.

"The bomb was placed inside a bag, which was left at the baggage counter of the store," he said. "The explosion was so powerful, a concrete wall of the store collapsed."

Most of the fatalities were employees of the cooperative, Ajirin said.

Chief Superintendent Akmad Mamalinta, a regional police chief, said the attack could have been conducted by new recruits of the al-Qaeda-linked Abu Sayyaf rebel group who were on a test mission.

"There are indications the attack was the handiwork of Abu Sayyaf rebels," he said. "They are our prime suspects."

A police investigator earlier said that authorities were looking at extortion as a possible motive for the attack. He did not give any other details.

Ajirin said US explosive experts were helping determine the type of explosive used in the attack on the two-storey building.

Police and soldiers have set up checkpoints around the downtown to prevent any other untoward incident.

Jolo is a stronghold of Abu Sayyaf separatists, who has been blamed for some of the worst terrorist attacks in the Philippines.

More than 2,000 US troops participated in monthlong joint military training exercises on the island, which ended in early March.