A bomb exploded outside a church in the southern Philippines during Sunday morning mass, killing five people and wounding 35, an army spokesman said, Reuters reported.

Rogue Muslim rebels were suspected of placing the bomb near a food stall outside the church in Cotabato City, said Colonel Jonathan Ponce. The device was detonated when an army truck was passing, he added.

"This is the handiwork of the rogue members of the Moro Islamic Liberation Front (MILF)," Ponce told reporters.

"The rebels are getting desperate and they are no longer choosing their targets. They are now attacking even places of worship."

A woman selling roast pork was killed on the spot while four others, including a soldier and three-year-old boy, died in a nearby hospital. Five soldiers were among those wounded, Ponce said.

Witnesses said the bishop celebrating the mass had just finished reading the gospel and was about to begin his homily when an explosion was heard.

"The explosion was so loud as if the cathedral was about to collapse," Merly Sandoval, a churchgoer, told a local radio station.

"It was like loud and frightening thunder. People inside the packed cathedral ran toward the altar. Outside, everybody was screaming, there was so much blood on the ground."

Ponce said the crude bomb, made from a mortar shell and remotely detonated by a mobile phone, was placed across the road from the church.

NOT A RELIGIOUS CONFLICT

Mohaqher Iqbal, a senior leader of the MILF, the largest Muslim rebel group in the mainly Roman Catholic Philippines, denied his group was involved in the attack.

"Who needs a Christian-Muslim conflict?," Iqbal told Reuters in a mobile phone text message.

"There's no religious conflict in the south. We're fighting for our right of self-determination. We're only defending our people and our communities."

However, rogue members of the MILF have been fighting the army since August, when the government ended peace talks with the MILF after the Supreme Court stopped a deal to expand an existing Muslim autonomous region on the southern island of Mindanao.

Nearly 600 people have been killed since then, many civilians caught in the fighting.

Fighting around the marshlands on central Mindanao has escalated in the last eight weeks, forcing more than 350,000 people to flee their homes and farms and pushing back any chance of resuming the peace talks.

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