MANILA — Senators on Sunday joined President Ferdinand Marcos Jr. and other officials in condemning the explosion that left at least 4 dead in Marawi, Lanao del Sur.

Senate President Juan Miguel Zubiri said he was "angered" and "appalled" by the blast that ripped through the Dimaporo gymnasium at the Mindanao State University's main campus on Sunday.

"To enact such violence on an innocent group of people gathered to hear Mass is completely heartless," he said, adding that it was "disheartening to see such violence play out in Marawi once again."

The Philippine military wrested back the ruined Marawi City after a five-month battle that claimed more than a thousand lives, after hundreds of pro-Islamic-State foreign and local fighters had seized it in May 2017.

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The blast that also injured scores of individuals occurred while they were attending a Catholic Mass. Authorities have yet to determine the motive or identify what explosive was used.

Zubiri said it was "alarming to see such a brazen attack on a state university" during the religious gathering. "No one should have to feel unsafe in places of learning and places of worship."

Senate Majority Leader Joel Villanueva said the fact that the blast happened inside a school that was supposed to be a safe place for everyone "makes it even more disturbing." He called on authorities to bring the perpetrators to justice.

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'STEP UP INTELLIGENCE'

Sen. Nancy Binay meanwhile called on the Philippine National Police and the Armed Forces of the Philippines to "step up their intelligence efforts" against lawless elements and terrorist groups after the deadly blast.

"We are all for peace in Mindanao, and we will not be cowed by threats or terror," she said.

Sen. JV Ejercito, for his part, cautioned that attacks like what happened in Marawi could risk "the very fabric of our community" if left unchecked.

He added: "Acts of atrocious extremism have no place in a society that values peace, harmony, and the pursuit of knowledge."

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BATO: PERPETRATORS 'NOT MUSLIMS NOR CHRISTIANS'

Sen. Ronald "Bato" dela Rosa believes that the perpetrators in the Marawi blast were "not Muslims nor Christians, they are terrorists!"

Conflict monitor Climate and Conflict Action Asia warned in a statement earlier Sunday that it is "too early to draw conclusions or to point fingers towards any individual or group."

The group called for calm "amidst attempts to polarize the situation and sow the seeds of a Muslim-Christian conflict" as it said that it has monitored hate speech on the ground and in social media in the aftermath of the blast.

Sen. Loren Legarda's statement meanwhile centered on her call for respect for religious beliefs

She noted that the bombing occurred during a religious gathering which underscored "the need for greater understanding and respect for diverse religious practices and beliefs."

For Sen. Win Gatchalian, "targeting a place of worship and endangering innocent lives is an appalling act that goes against the values of humanity."

Senators Grace Poe, Lito Lapid, Alan Peter Cayetano, Jinggoy Estrada, Imee Marcos, and Bong Revilla also expressed their condemnation of the deadly Sunday blast in separate statements.

Lanao del Sur and Maguindanao del Sur are part of the Bangsamoro Autonomous Region in Muslim Mindanao.

The country's Muslim minority were given fuller autonomy under former president Rodrigo Duterte as part of efforts to head off the lure of violent extremism.

Bangsamoro Government Chief Minister Ahod Ebrahim said he "condemned the atrocious and cowardly acts" and called for a "thorough investigation" into the incident.

Militant attacks on buses, Catholic churches, and public markets have been a feature of decades-long unrest in the region.

Manila signed a peace pact with the nation's largest rebel group, the Moro Islamic Liberation Front, in 2014, ending their deadly armed rebellion.

But smaller bands of Muslim fighters opposed to the peace deal remain, including militants professing allegiance to the Islamic State group.

Communist rebels also operate in the region.

—with a report from Agence France-Presse