BANGKOK >> A deadly bombing of a cathedral in the Philippines has brought fresh attention to the Islamic State’s ability to metastasize across the world, even as the militant group has been reduced to a sliver of turf in Syria.

The attack, consisting of two detonations, struck the Cathedral of Our Lady of Mount Carmel on the island of Jolo at the southern end of the Philippines, a region where Muslim insurgents have for decades battled the Catholic-majority state. At least 20 people were confirmed dead in the assault, which took place just as worshippers gathered for Mass on Sunday.

Through various online bulletins, the Islamic State, also known as ISIS, claimed responsibility.

The violence showcased the ability of the Islamic State to graft onto faraway militant movements and fan the flames of local conflicts by striking a high-profile target like a cathedral, the premier church in a Catholic diocese. Fighters from Indonesia, the Philippines and Malaysia flocked to Iraq and Syria in recent years, and returnees from the Islamic State’s battles have strengthened the reach and tactical power of extremist groups back in Southeast Asia.

The bombings in the Philippines occurred just after a Muslim-majority part of the island group of Mindanao, which includes Jolo, held a referendum on a delicate peace process. On Monday, the Philippine national security adviser, Hermogenes Esperon, implied that the bombings Sunday were most likely the work of rebels affiliated with Abu Sayyaf, a separatist militia with a stronghold in Jolo that is excluded from the current peace process.

Last week’s referendum on whether to create a Muslim autonomous region in Mindanao was approved by voters everywhere except on Jolo.

Through kidnappings, beheadings and the occasional bombing, Abu Sayyaf has terrorized the region for years, targeting foreigners and locals alike. A former leader of the militant group pledged allegiance to the Islamic State in 2014 and was later recognized as the Islamic State’s regional emir, but other factions of Abu Sayyaf have not publicly sworn their loyalty to the militant group.

President Rodrigo Duterte of the Philippines, who long served as mayor of Davao City, the capital of Mindanao, visited Jolo on Monday and directed security forces to “crush” the militants, according to Delfin Lorenzana, the country’s defense secretary.

Lorenzana, who traveled to Jolo with the president, said that the police considered six people seen in video footage of the cathedral to be suspects. Esperon, the national security adviser, said that those six people had been identified by intelligence sources as possibly the sons or relatives of known Abu Sayyaf operatives.

The earliest high-profile extremist to pledge allegiance to the Islamic State in the Philippines was a former leader of Abu Sayyaf, Isnilon Hapilon. After declaring his loyalty in 2014, Hapilon united ragtag rebels from the jungle, university-educated ideologues and even former Catholics with the particular zeal of the converted.

In 2017, his coalition laid siege to the Mindanao city of Marawi. The Philippine military operation to extract the extremists decimated much of the city and left more than 1,200 people dead, including Hapilon and a legion of foreign fighters.

Ahmad El-Muhammady, a counterterrorism analyst at the International Islamic University Malaysia, who has conducted extensive interviews with jailed Islamic State militants in Malaysia, said that such groups “may be locally created, but they are regionally networked and globally inspired by ISIS.”

“For Mindanao, ISIS is coming along at the right time, and each side is benefiting from the other,” he added. “ISIS can say, ‘We have global reach,’ and local groups, like Abu Sayyaf factions, can sit on the shoulder of a giant like ISIS and get connections and financial support.”

As Philippine investigators sifted through the debris of the devastated cathedral — shattered pews and crushed crucifixes — a statement from the Islamic State said that suicide bombers had carried out the attacks.

Today, however, the chief of the Philippine National Police, Oscar Albayalde, said from Jolo that the explosions, which he described as occurring 12 to 15 seconds apart, had probably been detonated by cellphone. The casualty list released by the Philippine authorities is not believed to include the names of anyone suspected of being a bomber.

Suicide bombing is a rarity in the Philippines. The first case in recent history occurred in July on the Mindanao island of Basilan, a stronghold of Abu Sayyaf rebels who have pledged loyalty to the Islamic State.

The Islamic State claimed responsibility for that suicide attack, too, in which a Moroccan national detonated an explosive-filled van at a checkpoint, killing nine security officers and bystanders. Philippine authorities have denied that that assault had any link with the Islamic State, however.

For years, officials in the Philippines have been reluctant to acknowledge that the Islamic State has taken root in the country, even as the militant group has taken credit for a series of deadly attacks across Southeast Asia and been implicated in other plots foiled by regional governments.

In 2015, a year after formally announcing its caliphate, the Islamic State issued an anniversary video in which it revealed that 16 of the 35 “provinces” of its state were located outside Iraq and Syria. One was in the Philippines.

Southeast Asian counterterrorism experts note that Islamic State literature often glorifies the exploits of militants from Mindanao.

“The Filipino government downplays the ideological element of pro-ISIS coalitions to its detriment,” said Sidney Jones, the director of the Institute for Policy Analysis of Conflict in Jakarta, Indonesia, who has tracked the rise of the Islamic State and other militant groups in Southeast Asia.

“There are personal rivalries and clan money that drive what’s happening in Mindanao but to discount ISIS totally is dangerous because it ignores something that is happening across the region,” she added. “When ISIS says it has bombed a church in the Philippines, I think it’s part of an effort to clearly target something big and symbolic and say, ‘We’re here.’”

The first assaults claimed by the Islamic State in Southeast Asia occurred in January 2016, when suicide attackers mounted deadly raids near a Starbucks in central Jakarta.

Indonesia, home to the world’s largest Muslim population, had endured terror attacks before, most notably the bombing of a nightclub in Bali that killed more than 200 people in 2002. That attack and others, including the bombings of Western-owned hotels and of the Australian Embassy in Jakarta, were orchestrated by the homegrown radical group Jemaah Islamiyah, which aligned itself with al-Qaida.

But just as al-Qaida lost strength elsewhere in the world, Jemaah Islamiyah was supplanted in Indonesia by other militant groups that pledged allegiance to the Islamic State. In one of the most shocking episodes, an entire family — mother, father, two teenage sons and two younger daughters — blew themselves up in back-to-back attacks on three churches in Surabaya, the second largest city in Indonesia, in May last year.

The Islamic State claimed responsibility for those assaults, which killed 12 people, describing them as a “martyrdom operation.”

Within another day, two other families in Surabaya fatally set off their own bombs, one at a police station and another in a premature misfire in their own apartment.

All three families appeared to be linked to Jamaah Ansharut Daulah, an Indonesian militant organization that has pledged allegiance to the Islamic State. Since then, about 300 people believed to be connected to Jamaah Ansharut Daulah have been rounded up by Indonesian counterterrorism forces.

Surabaya is a multiethnic, multifaith city, in a country run by a secular government. The southern tip of the Philippines, while majority Muslim, has a significant Catholic minority. And Malaysia, from where at least 130 people, according to Malaysian counterterrorism experts, have traveled to Iraq and Syria to join the Islamic State, balances a Malay Muslim majority with large ethnic Chinese and Indian populations.

Yet militants in all three countries have over the decades called for those sharing an ethnic Malay heritage to create a crescent-moon-shape caliphate across the region. Other extremists have expressed more isolated concerns for local autonomy.

Southeast Asian militants gained battlefield experience in Afghanistan and ideological guidance in Pakistan, Saudi Arabia and Yemen. More recently, hundreds of Filipinos, Indonesians and Malaysians traveled to Iraq and Syria to act as foreign fighters for the Islamic State, forming their own corps with its own recruitment tools online.

“We have to look at these extremist movements in the region without dividing them by national borders,” said Badrul Hisham Ismail, program director for IMAN Research, a security-focused think-tank in Kuala Lumpur, Malaysia, who has studied violent extremism.

“A top-down, nation-by-nation approach doesn’t work,” he added.

In June 2016, the Islamic State confirmed in a video that Hapilon, the former Abu Sayyaf commander, was the leader of the group in Southeast Asia. The same video called for those who could not travel to Syria to instead make their way to the southern Philippines, Jones of the Institute for Policy Analysis of Conflict said. Some did, dying the next year in the battle for Marawi.

Making a cameo in the same video was Abu Walid, an Indonesian who spent nine years in detention in the Philippines before turning up in Syria and becoming close to the leadership of Islamic State, according to the U.S. Treasury Department, which placed sanctions on him. On Wednesday, Abu Walid, the nom de guerre of Mohammed Yusop Karim Faiz, was killed in fighting in Syria.

“So he’s dead, but then you have the Jolo church bombing a few days later,” Jones said. “This doesn’t look like it’s going away.”

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