The Islamic State of Iraq and Syria (ISIS) claimed responsibility Sunday for a pair of bombings at a Catholic cathedral on a southern Philippine island that killed at least 20 people and wounded 81. The militant group's news agency Amaq took responsibility for the blasts, according to Reuters.

Security officials say the first bomb went off in or near the Jolo cathedral during Sunday Mass, followed by a second blast outside the compound as government forces were responding to the attack.

Jolo Island has long been troubled by the presence of Abu Sayyaf militants, who are blacklisted by the U.S. and the Philippines as a terrorist organization because of bombings, kidnappings and beheadings.

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Images posted on social media showed the main road leading to the church sealed off by soldiers in armored personnel carriers, BBC News reports.

Defense Secretary Delfin Lorenzana called the attack a "dastardly act" and said "we will use the full force of the law to bring to justice the perpetrators behind this incident."

Philippine Army members secure the area outside a church after a bombing attack in Jolo, Sulu province, Philippines January 27, 2019. Armed Forces Of The Philippines - Western Mindanao Command/Handout via Reuters

Pope Francis, in Panama for World Youth Day, expressed "firmest reproach" for the twin bombings and called for those responsible to change their ways.

He also prayed for the dead and wounded during his Sunday blessing. The pope said the "terrorist attack" was just the latest to hit the Christian community in the Philippines.