At least 27 people have been killed and more than 70 injured after two bombs were detonated outside a Roman Catholic cathedral on a southern Philippine island.

The first bomb went off in or near the Jolo cathedral in the provincial capital during Sunday mass.

A second blast occurred outside the compound as government forces were responding to the attack, security officials said.

Police said at least 27 people died and 77 were wounded.

The scene inside the cathedral in Jolo
The scene inside the cathedral in Jolo (WESMINCOM Armed Forces of the Philippines/AP)

The dead were 20 civilians and seven soldiers, and the wounded were 14 troops, two police and 61 civilians.

Photos on social media showed debris and bodies lying on a busy street outside the Cathedral of Our Lady of Mount Carmel, which has been hit by bombs in the past.

Troops in armoured carriers sealed off the main road leading to the church while vehicles were transporting the dead and wounded to the hospital.

Some casualties were evacuated by air to nearby Zamboanga city.

Casualties receive treatment in hospital
Casualties receive treatment in hospital (WESMINCOM Armed Forces of the Philippines/AP)

“I have directed our troops to heighten their alert level, secure all places of worships and public places at once, and initiate pro-active security measures to thwart hostile plans,” said Defence Secretary Delfin Lorenzana in a statement.

Jolo island has long been troubled by the presence of Abu Sayyaf Islamic militants, who are blacklisted by the United States and the Philippines as a terrorist organisation because of years of bombings, kidnappings and beheadings.

No-one immediately claimed responsibility for the attack.

It came nearly a week after minority Muslims in the predominantly Roman Catholic nation endorsed a new autonomous region in the southern Philippines in hopes of ending nearly five decades of a separatist rebellion that has left 150,000 people dead.

Although most of the Muslim areas approved the autonomy deal, voters in Sulu province, where Jolo is located, 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.

Western governments have welcomed the autonomy pact.

They worry that small numbers of Islamic State-linked militants from the Middle East and Southeast Asia could forge an alliance with Filipino insurgents and turn the south into a breeding ground for extremists.