MANILA - At least 20 people were killed and dozens injured early on Sunday (Jan 27) morning when two bombs exploded at a Roman Catholic cathedral in the mainly Muslim province of Sulu.

It is 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-wracked 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.

Military spokesman Brigadier-General Edgard Arevalo tallied 17 dead as of 11am, including five soldiers sent to secure the cathedral.

Just before noon, the number climbed to at least 19 killed, police chief Director-General Oscar Albayalde said, citing unconfirmed reports from his field commanders.

Hours later, Chief Superintendent Graciano Mijares, police director in the region that covers Sulu, said in a radio interview that 27 had already died.

He revised that late in the afternoon to 20, with 81 wounded, including 14 soldiers and two men from the Coast Guard. His spokesman said there was “miscommunication” with the hospitals who took in the dead and wounded.

Images on social media and photos released by the military showed debris and bodies lying on a busy street outside the cathedral, which had been hit by bombs in the past.

Troops in armoured carriers sealed off the main road leading to the church, while vehicles transported the dead and wounded to the hospital. The military said it airlifted some of the wounded to the nearby city of Zamboanga for medical treatment.

President Rodrigo Duterte’s spokesman Salvador Panelo said in a text message: “The enemies of the state have boldly challenged the capability of the government to secure the safety of the citizenry in that region.

"The (military) will rise to the challenge and crush these godless criminals... We will pursue to the ends of the earth the ruthless perpetrators behind this dastardly crime until every killer is brought to justice and put behind bars.

"The law will give them no mercy,” he said.

Defence Secretary Delfin Lorenzana said in a statement that he had “directed our troops to heighten their alert level, secure all places of worship and public places at once, and initiate proactive security measures to thwart hostile plans”.

General Benjamin Madrigal, the military chief, called the attack a “treachery”, and said he had already ordered security forces “to pursue the criminals with the full force of the law”.

No one has immediately claimed responsibility for the attack.

“We don’t have any lead yet. We’re looking at different threat groups in the area. We can’t say yet if this has anything to do with the recent plebiscite,” said Mr Albayalde.

The attack came nearly a week after more than 1.5 million Muslims, a minority in the predominantly Catholic nation, overwhelmingly approved a more powerful autonomous region in the Philippines’ south.

They had voted for the new, self-administered region called the “Bangsamoro”, or nation of Moros, in the war-torn island of Mindanao, in hopes of ending nearly five decades of a separatist rebellion that has left over 150,000 people dead.

The Bangsamoro law is the product of years of often tumultuous negotiations between the government in Manila and the Moro Islamic Liberation Front (MILF), the main rebel group.

Voters in Sulu, however, rejected it.

The province is home to a rival rebel faction that is opposed to the deal, as well as the Abu Sayyaf group, which is not part of any peace process.

The Abu Sayyaf, a gang of self-styled Islamic militants founded in the 1990s with seed money from the Al-Qaeda network, has long used Sulu as a base, carrying out kidnappings and bombings.

It has pledged allegiance to the Islamic State in Iraq and Syria (ISIS).

Though Abu Sayyaf officially has a separatist Islamist agenda, it has capitalised on decades of instability in Mindanao to generate tens of millions of dollars from piracy and ransom payments.

The group is still holding at least five hostages - a Dutch national, two Malaysians, an Indonesian and a Filipino - in their jungle bases mostly near Sulu’s Patikul town, not far from Jolo.

The Abu Sayyaf is believed to have carried out the worst terror attacks in Philippine history, including the bombing of a passenger ferry in Manila Bay that killed more than 100 people in 2004.

Mr Mujiv Hataman, governor of the current Autonomous Region in Muslim Mindanao that the Bangsamoro region will replace by 2022, told the online news site Rappler he was “99 per cent certain” the Abu Sayyaf was behind Sunday’s attack.

The government is hoping the new Bangsamoro region, with former top rebels from the MILF at the helm, can rein in a new wave of Islamist radicals sweeping across Mindanao.