Philippine forces are on high alert after a bomb killed four people and wounded several others during a Catholic Mass in a university gymnasium in the south of the country, an attack authorities have called Islamist terrorism.

“I condemn in the strongest possible terms the senseless and most heinous acts perpetrated by foreign terrorists,” President Ferdinand Marcos Jr said on Sunday.

“Extremists who wield violence against the innocent will always be regarded as enemies to our society.”

The blast in Marawi, a city besieged by pro-Islamic State militants for five months in 2017, followed the killing of 11 combatants in a military operation on Friday some 200 kilometres away in Maguindanao del Sur.

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Urging calm, Marcos posted on social media platform X that he had instructed the national police and armed forces “to ensure the protection and safety of civilians and the security of affected and vulnerable communities”.

“Rest assured we will bring the perpetrators of this ruthless act to justice.”

Army Major General Gabriel Viray III called the attack at the Mindanao State University “a terror act”, speaking to reporters as explosive disposal experts were deployed.

“Right now we are on heightened alert and our troops remain vigilant as we are determining the motive and identifying the perpetrators to really ascertain who was behind it,” Viray said.

Military officials surveyed the gymnasium, which appeared intact except for burn marks in the centre where the explosion occurred, according to images shared by the government of Lanao del Sur on its Facebook account.

White plastic chairs were strewn about the scene.

Videos posted on X by DZBB radio showed rescuers carrying injured people out of the gym on plastic chairs.

“Terroristic attacks on educational institutions must also be condemned because these are places that promote the culture of peace,” Lanao del Sur Governor Mamintal Adiong Jr said in a statement.

The Philippine military killed 11 militants on Saturday, including members of the Dawlah Islamiyah-Philippines group, in an operation that recovered 10 high-powered firearms and three explosive devices.

“We are looking at the bomb signature to determine if the group was indeed behind it,” Viray said.

The coast guard said in a statement it had directed its districts to intensify pre-departure inspection at ports.

Mindanao State University is “deeply saddened and appalled by the act of violence that occurred during a religious gathering”, it said in a statement on Facebook.

“We unequivocally condemn in the strongest possible terms this senseless and horrific act.”

The university said it was suspending classes until further notice.

-Reuters, with AP