Indonesian police say two suicide bombers believed to be members of a militant network that pledged allegiance to the Islamic State are to blame for an attack that injured 20 people outside a church in South Sulawesi province on Palm Sunday.

The two attackers blew themselves up outside the Sacred Heart of Jesus Cathedral in the provincial capital Makassar after trying to enter the main gate of the church just as the mass was ending.

They had been on a motorcycle when they were stopped by security guards, national police spokesman Argo Yuwono said.

South Sulawesi police chief Merdiansyah said the morning service had finished and worshippers were walking out of the church when the two attackers detonated their explosives, resulting in an explosion of “high intensity”.

On Monday morning (Australian time), the number of people injured went up from 14 to 20.

The most recent people found to be wounded included four security guards and several worshippers.

“We see that there are victims and parts of human bodies have been torn apart,” South Sulawesi police spokesman E. Zulpan told Reuters.

Police said both attackers were killed instantly and evidence collected at the scene indicated one of the two was a woman.

National Police Chief General Listyo Sigit Prabowo told reporters when he visited the crime scene late on Sunday that the two attackers are believed to have been members of the militant group Jemaah Anshorut Daulah, which has pledged allegiance to Islamic State group and was responsible for deadly suicide bombings on Indonesian churches in 2018.

He said one of the attackers was believed to have links to a church bombing in the Philippines.

The attack a week before Easter in the world’s most populous Muslim-majority nation came as the country was on high alert following December’s arrest of the leader of the Southeast Asian militant group, Jemaah Islamiah, which has been designated a terror group by many nations.

Indonesia has been battling militants since bombings on the resort island of Bali in 2002 killed 202 people, mostly foreign tourists. Attacks aimed at foreigners have been largely replaced in recent years by smaller, less deadly strikes targeting the government, police and anti-terrorism forces and people militants consider as infidels.

Police have identified one of Sunday’s attackers only by his initial, L, who they believe was connected to a 2019 suicide attack that killed 23 people at Our Lady of Mount Carmel Cathedral in the Philippine province of Sulu, Prabowo said.

He said the two attackers were linked to a group of suspected militants arrested in Makassar on January 6, when a police counterterrorism squad shot and killed two suspected militants and arrested 19 others.

Members of the squad were initially supposed to arrest the two slain men for their alleged role in the Philippine suicide bombing.

He said on Sunday police arrested four suspected militants believed to have links with the attackers in a raid in Bima, a city on Sumbawa island in East Nusa Tenggara province.

“We are still searching other members of the group and I have ordered the Densus 88 to pursue their movement,” Prabowo said, referring to Indonesia’s elite police counterterrorism squad.

Chief security official Mahfud MD said authorities expected to recover more wounded victims.

The government had ordered security forces to beef up security measures in houses of worships and other public facilities, he added.

President Joko Widodo and the Indonesian Council of Muslim Scholars spoke out against the attack.

“I strongly condemn this act of terrorism and I have ordered the police chief to thoroughly investigate the perpetrators’ networks and tear down the networks to their roots,” Mr Widodo said in an online broadcast following the attack.

Council deputy chairman Anwar Abbas said in a statement that the attack was a heinous act that violated the tenets of any religion.

“We call on authorities to apprehend the perpetrators and mastermind of the attack,” he added.

The Indonesian Communion of Churches urged Christians to remain calm.

“I call on all members of the community not to be afraid and restless,” chairman Gomar Gultom said.

“Please pray for our safety.”

-with AAP