Police and military personnel look at debris from an explosion scene inside a church on the island of Jolo in Sulu Province, the Philippines, Jan. 27, 2019. (Xinhua/Stringer)

MANILA, Jan. 28 (Xinhua) -- Jolo in southern Philippine Sulu province is on lockdown as security forces investigated the twin explosions at a Roman Catholic cathedral that killed 20 people and wounded over 100 on Sunday, authorities said on Monday.

The Armed Forces of the Philippines (AFP) and the Philippine National Police (PNP) are on heightened alert after two improvised explosive devices (IEDs) went off Sunday at the Cathedral of Our Lady of Mount Carmel on the island of Jolo in southern Philippine province of Sulu.

Philippine Defense Secretary Delfin Lorenzana said President Rodrigo Duterte ordered the military to crush the Abu Sayyaf for all the atrocities they committed against the people.

Philippine authorities blamed the attack on the Abu Sayyaf Group, a bunch of up to 400 violent extremists behind the series of bombings, kidnappings, and beheadings in Western Mindanao in southern Philippines. The group is based on the island provinces of Basilan and Sulu.

Authorities are hunting at least six people who were seen on surveillance cameras around the church.

Duterte is outraged by the attack, presidential spokesman Salvador Panelo said, adding that Duterte expressed disappointment that despite the movement to tread the path towards peace and development in Mindanao, there are still forces that sow terror and kill people.

"He has issued a directive to the AFP to secure the safety of the citizenry and to attend to the injured and provide support to the families of the victims," Panelo told a news conference at the presidential palace.

Duterte visited Jolo on Monday. He was seen inspecting the bloodied floor of the church strewn with mangled wooded pews and shredded ceiling.

PNP Director General Oscar Albayalde told reporters at a new conference in Jolo that there are no indications that the attack was carried out by a suicide bomber.

Based on initial investigation, he said the bombers used pipe bombs stuffed with about two kg of explosives to achieve a maximum effect of lethality and terror.

Albayalde added that checkpoints were set up to "contain" the area. Stores were closed and people are told to keep off the streets.

Authorities have declared a heightened alert status to ensure that no similar attacks will break out in other parts of the country.

Under heightened alert, police regional directors nationwide are mandated to intensify their security measures.

Jefferson Nadua, a priest assigned in the cathedral, said he received reports that a teenager critically wounded in the blasts died in the hospital in Zamboanga City where the victim was airlifted on last Sunday.

Nadua said that there were an estimated 100 to 120 churchgoers inside and at the entrance of the church when the explosions took place while a celebration was underway.

"It's the worst (attack) so far. This is a call for us to be united as people, to be one and to cry out for justice," he told a local television interview.

It was not the first time that the cathedral was attacked, according to Nadua, adding that there were a number of similar attack in the past.