The suspects in Sunday's bomb attack on an Indonesian cathedral were not long married, the Associated Press reports. 

At least 20 people were injured when a couple let off pressure cooker bombs at the entrance to the Roman Catholic Sacred Heart of Jesus Cathedral in Makassar, on Sulawesi.

It happened as worshippers were leaving a Palm Sunday service to mark the start of Holy Week in the approach to Easter.  

The couple, indentified by policy only as L and YSF, had been married for six months and were local to Makassar. Neighbours told AP they were between 23- and 26-years-old. 

The Communion of Churches in Indonesia (PGI) has expressed its "deep sorrow" over Sunday morning's "barbaric" attack. 

"This ill-fated incident adds to the long list of acts of violence and terror that have occurred in the archipelago," it said.

The organization is also urging Christians to remain calm and step up security at their churches as they prepare to celebrate Easter. 

"I urge all people to remain calm and fully entrust the handling of this problem to the relevant authorities. I call on all people not to be afraid and anxious, but to remain vigilant," said PGI chair Gomar Gultom.

"I fully believe that our apparatus is able to thoroughly investigate this case and can create a safe and comfortable atmosphere for the Indonesian people, especially the people of Makassar." 

He urged Christians to follow the peaceful example set by Jesus who "did not come with power, strength, much less violence to fight and destroy lives."

"It is in such a spirit of gentleness that I invite Christians to face this event, while praying for the peace of our society," he said. 

The attackers detonated their devices when they were approached by security guards and were reported to be the only fatalities in the attack, which has been condemned by Indonesian president Joko Widodo. 

They are believed to have had ties to the Islamic State-linked extremist group, Jemaah Anshorut Daulah, which is behind past suicide bombings in Indonesia, including a 2016 attack on a Starbucks that killed four, and an attack on a church in Kalimantan the following year in which a 2-year-old girl died. 

Police are investigating possible links between one of Sunday's attackers and a 2019 suicide attack on Our Lady of Mount Carmel Cathedral in Jolo, Sulu, in the Philippines.

In the wake of Sunday's attack, security has been tightened at churches across Indonesia as they prepare to celebrate Easter.

International Christian Concern (ICC) reports that the authorities have asked churches to hold their Easter services virtually instead. 

Gina Goh, ICC's Regional Manager for Southeast Asia, said, "As Christians around the world prepare themselves for the Holy Week, it is pure evil that the terrorists deliberately chose this time to attack and inflict suffering on Christians.

"We ask for prayers for the wounded and urge the Indonesian authorities to investigate and arrest the terrorists responsible for this attack.

"Terrorism should not be allowed to exist in a country that upholds Pancasila [the guiding philosophy of the Indonesian state]."