Philippines president Rodrigo Duterte ordered his country's armed forces to crush the ISIS militants believed to be behind the bombings at a Catholic cathedral that killed at least 20 people and wounded more than 100 on Sunday.

Duterte and his top security officials on Monday visited the site of the bombing, Cathedral of Our Lady of Mount Carmel, in the southern Sulu provincial capital on Jolo island. He reportedly walked slowly into the bombed cathedral, where the wooden pews were still in disarray less than a day after twin explosions were set off.

The first blast sent people, some of them wounded, running out the church's main door. Army troops and police officers were rushing inside when the second bomb exploded a minute later.

“This is an act of terrorism,” said Defense Secretary Delfin Lorenzana, who was with the president. “This is not a religious war.”

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Lorenzana blamed the attack on Abu Sayyaf militants who have carried out years of bombings, kidnappings, and beheadings in Jolo island. The militants have aligned themselves with ISIS, which claimed responsibility for the attack.

Abu Sayyaf has an estimated 300 to 400 members, mostly in Sulu, where it is holding several foreign and Filipino kidnap victims.

Duterte later met with families of the victims at a military camp in Jolo, where coffins were laid side by side.

Police around the country were on heightened alert to prevent similar attacks.

The bombings came nearly a week after minority Muslims in the predominantly Roman Catholic nation endorsed a new autonomous region in the southern Philippines in hopes of ending nearly five decades of a separatist rebellion that has left 150,000 people dead. Although most Muslim areas approved the autonomy deal, voters in Sulu province rejected it.

A statement by ISIS posted on social media claimed the attack was carried out by two suicide bombers who wore explosive belts, one detonating at the gate and the other in the parking lot.

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Police said at least 20 people -- 15 civilians and five troops -- died and 111 people -- including about 90 civilians -- were wounded

The Associated Press contributed to this report.