At least 20 people were killed and dozens injured yesterday when two bombs exploded at a Roman Catholic cathedral in the mainly Muslim province of Sulu.

It was one of the deadliest attacks in recent years in the Philippines' restive south.

The first bomb went off at around 8am, during a mass inside the Cathedral of Our Lady of Mount Carmel in Jolo, a poverty-racked island of some 700,000.

A second blast, purportedly from a bomb stashed inside a motorcycle, followed minutes later at a carpark outside the compound, as those inside the cathedral were fleeing and as government forces were responding to the attack.

Chief Superintendent Graciano Mijares, police director in the region that covers Sulu, said in a report that at least 20 people had died, with over 100 wounded, including 14 soldiers and two men from the Coast Guard.

No one immediately claimed responsibility for the attack.

But Mr Mujiv Hataman, governor of the Autonomous Region in Muslim Mindanao (ARMM) that covers Sulu, told the online news site Rappler he was "99 per cent certain" the Abu Sayyaf, a gang of self-styled Islamist militants founded in the 1990s with seed money from the Al-Qaeda network, was behind the blasts.

The attack came nearly a week after more than 1.5 million Muslims voted for a new, more powerful, self-administered region, in hopes of ending nearly five decades of a separatist rebellion and a new wave of Islamist extremism sweeping across war-torn Mindanao island.

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