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 worshipers 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 on 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.