MANILA - A crude bomb exploded in front of a petrol station on a restive island in the southern Philippines on Tuesday, killing six people and wounding 40, a military spokesman said.

The bomb was concealed in a motorcycle parked across the road from a Roman Catholic church on the island of Jolo, a stronghold of Muslim rebel groups linked to the Jemaah Islamiah (JI) network, said Lieutenant-Colonel Edgard Arevalo.

"Two men were killed on the spot," Arevalo told reporters, adding four others died later at the hospital. He said three police officers were among some 40 people wounded.

"Many of the wounded are in critical condition," he said.

A second crude bomb was found near the steps of Mount Carmel church in Jolo town across from the petrol station and was safely detonated by troops, Arevalo said. Another suspected package was discovered at a store about a block away from the church.

No group has claimed responsibility for the attack. But security forces suspected it was the work of the Abu Sayyaf, a small Muslim rebel group with links to the regional militant network Jemaah Islamiah.

The Jolo attack comes two days after a crude bomb exploded outside a cathedral in Cotabato City on the main island of Mindanao, killing five and wounding 55.

The mainly Roman Catholic Philippines has been searching for an Italian engineer working for the International Committee of the Red Cross on Jolo island, held for nearly six months by the Abu Sayyaf.

The 40-year Muslim separatist conflict in the south is driving away potential investments into the impoverished region, believed to be sitting on rich deposits of minerals, oil and natural gas.

(Reporting by Manny Mogato; Editing by Rosemarie Francisco and Alex Richardson)