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MANILA - The Islamic State (IS) militant group has claimed responsibility for a double bomb attack that killed and injured dozens at a Catholic church in Jolo, Sulu, a US-based group that monitors jihadist websites said Monday.

The IS issued a formal communique saying 2 suicide bombers detonated explosive belts at the Jolo Cathedral, according to the SITE Intelligence Group.

The military is not discounting this claim, but initial assessment found that the bombs were planted at the church, not strapped onto suicide bombers, said Western Mindanao Command chief Lt. Gen. Arnel Dela Vega.

The first bomb on Sunday went off during a Mass, leaving bodies strewn across the cathedral where pews and windows shattered. A second bomb exploded at the parking area moments later as troops rushed to help the wounded.

• IN PHOTOS: Bloody Sunday bombing in Jolo

A military report said the second bomb was left in the utility box of a motorcycle in the parking area.

Before the IS' claim of responsibility, the military earlier said the Abu Sayyaf could be behind the blasts.

However, both pipe bombs used in the attack contained ammonium nitrate fuel, which is not common in explosives from previous Abu Sayyaf attacks, said Dela Vega.

Investigators are looking into the possibility that foreigners made the bomb or taught the Abu Sayyaf how to assemble them, he said.

Authorities also identified several persons of interest that acted suspiciously minutes before the attack, he added.

National police chief Director General Oscar Albayalde meanwhile said many other threat groups could be behind the bombings. Those behind the attack may be identified once investigators reconstruct the explosives, he said.

The bombings have left 20 dead and 112 wounded as of 7:30 p.m. Sunday, said command incident spokesperson Faghry Ukkoh.

The bloodshed came less than a week after voters' decisive approval of giving Muslims in the south more control over their own affairs, which sparked hopes of quelling long-time separatist violence.

Sulu voted against the Bangsamoro region, but will still be included in the area because voters from across the current autonomous region approved it.

The province's remote Jolo island is a base of the Abu Sayyaf, which was not part of the decades-long peace process with the nation's largest separatist group, Moro Islamic Liberation Front, whose 2014 deal with the government led to the Bangsamoro region's creation.

Abu Sayyaf has been blamed for deadly bombings, including an attack on a ferry in Manila Bay in 2004 that claimed 116 lives in the country's deadliest terror assault.

• Ferry bomb to Marawi siege: Philippines' worst attacks

It has also kidnapped many foreigners and Filipino Christians, hiding them in the jungles of Jolo and other southern islands.

The whole Mindanao is under martial law until the end of 2019.

President Rodrigo Duterte first declared military rule in the south when firefights erupted between state troops and terrorists in Marawi City in May 2017.

With reports from Agence France-Presse; Chiara Zambrano, ABS-CBN news