Philippine forces were on high alert after a bomb killed four people and wounded many more during a Catholic mass in a university gymnasium in the south of the country, an attack the authorities called Islamist terrorism. “I condemn in the strongest possible terms the senseless and most heinous acts perpetrated by foreign terrorists,” said the president, Ferdinand Marcos Jr. “Extremists who wield violence against the innocent will always be regarded as enemies to our society.” Later on Sunday, the Islamic State group claimed responsibility for the attack. The group wrote on Telegram that its members detonated a bomb in the gathering. 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 in Maguindanao del Sur, 200km away. Urging calm, Marcos posted on the 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.” Regional military commander Maj Gen Gabriel Viray III said at least four people were killed by the explosion, including three women, and 50 others were brought to two hospitals for treatment of mostly minor injuries. The explosion caused panic among dozens of worshippers and left the victims bloodied and sprawled on the ground, said Taha Mandangan, the security chief of the sprawling state-run university campus. At least two of the wounded were fighting for their lives, Mandangan said. Army troops and police immediately cordoned off the area and were conducting an initial investigation and checking security cameras for any indication of who may have been responsible for the attack. Security checkpoints were set up around city. Viray called the attack “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. Videos posted by DZBB radio on X 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,” the 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. Mindanao State University said in a statement on Facebook it was “deeply saddened and appalled by the act of violence that occurred during a religious gathering. We unequivocally condemn in the strongest possible terms this senseless and horrific act.” The university said it was suspending classes until further notice.