The Philippine interior minister said an Indonesian couple with ties to an Islamic State-linked group are behind a church bombing that killed 20 people last weekend. 

The Christian community in the Philippines was badly shaken by the attack on the cathedral on the mainly Muslim island of Jolo, in the country's southern Sulu province.  

The Islamic State claimed responsibility for the attack, which came days after a peaceful referendum that resulted in a vote favouring autonomy for majority Muslim parts of the Mindanao.  

The island region of Mindanao has been under martial law since May 2017 when terrorists affiliated with ISIS stormed Marawi City and engaged in a five-month standoff with government forces.

On Friday, Interior Secretary Eduardo Año said the evidence, including eye witness accounts, pointed to an Indonesian man and his wife being behind Sunday's church attack.

'They are Indonesians,' Año, a former military chief, told CNN Philippines' News Night. 'I am certain that they are Indonesians.'

He said the pair had been helped by radical Islamist militant group Abu Sayyaf.

'There are two foreigners involved in the bombing and they were aided by local Abu Sayyaf who acted as guide, and probably conducted preliminary surveillance prior to the bombing,' Año said.

Philippine President Rodrigo Duterte said it appeared to be a suicide bombing, which if confirmed would make it one of the first in the country.

On Wednesday, a deadly grenade attack on a mosque in the largely Christian province of Zamboanga, in the south-west of the Philippines, killed two people. It is unclear who carried out the attack.

Zamboanga is close to Jolo, a stronghold of Abu Sayyaf, a group notorious for its kidnappings.