COTABATO - Foreign Islamic militants may have had a link to a bombing outside a church in the southern Philippines that killed five people and injured dozens, a military spokesman said Monday.

Major Randolph Cabangbang said investigators were probing whether Jemaah Islamiyah (JI), the group behind the Bali bombings, may have been involved in the blast outside the Immaculate Conception Roman Catholic cathedral.

The military also warned that there could be follow-up bombings across the southern Philippines island of Mindanao and said extra security measures would be put in place.

Cabangbang told reporters a number of JI militants are "here in the country."

Known members of the group Dulmatin, Umar Patek, and Zulkifli bin Hir are thought to be operating in the southern Philippines, he said, but added that there was no proof as yet they were involved.

"There's an ongoing operation to capture those terrorists," he added.

Philippines-based US military advisers are helping with the investigation, Cabangbang added.

US ambassador to the Philippines Kristie Kenney, visiting the southern Philippines port of Zamboanga, told reporters she hoped those responsible for the blast "be brought to justice."

But she declined to comment on any US involvement in the investigation.

The Philippine military believe JI militants in the Philippines have given bomb-making training to another Islamic separatist group, the Moro Islamic Liberation Front (MILF).

The military alleges the MILF is behind previous bombings of Christian targets in the southern Philippines.

But the MILF denied any involvement in Sunday's bombing.

"We are not responsible for the bombing," MILF spokesman Eid Kabalu told reporters. "We even condemn this attack."

Police on Monday raised the number of casualties to five dead and 38 wounded. Three of the casualties, including one of the dead, are soldiers, a police statement said.

Philippine Army spokesman Lieutenant-Colonel Romeo Brawner warned: "There is a big possibility that there will be more bombings."

Citing military intelligence reports, Brawner said previous bomb attacks this year that have hit several Mindanao cities "are all connected to each other."

The objective appears to be to "divert our troops and prevent them concentrating government resources" towards the arrest of two hardline MILF leaders, Ameril Umbra Kato and Abdurahman Macapaar, he added.

The two are accused of leading a series of deadly raids on Christian communities across Mindanao last year that left dozens of civilians dead and more than half a million displaced.

Meanwhile, police are interrogating a man arrested by the military at the blast site on Sunday.

Rico Aburva, a 34 year-old resident of nearby Sultan Kudarat town, is accused of planting the improvised explosive device, said Chief Superintendent Josefino Cataluna, the regional police chief.

"The suspect was trapped because he's not familiar (with) the city," said Lieutenant-Colonel Jonathan Ponce, spokesman for an army division based in Cotabato.