An explosion has ripped through a university gymnasium in the southern Philippines during a Catholic mass in what authorities strongly suspect was an attack by Muslim separatists.

Three people were killed at the Sunday morning service and at least nine others wounded, police say.

The authorities are investigating Sunday’s explosion at Mindanao State University, regional police director Allan Nobleza, told reporters, adding that “revenge” by pro-Islamic State militants was a prime angle being pursued .

The blast occurred in Marawi, a city besieged by Islamist militants for five months in 2017.

The Philippine military said on Saturday they had killed 11 militants, including members of the Dawlah Islamiyah-Philippines, a pro-Islamic State group, in a military operation the day before in Maguindanao del Sur province.

“I condemn the violent bombing incident that transpired this morning,” Lanao del Sur Governor Mamintal Adiong Jr said in a statement.

“Terroristic attacks on educational institutions must also be condemned because these are places that promote the culture of peace.”

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 said it was suspending classes until further notice.

-AAP