Philippine forces were on high alert on Sunday 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 the authorities called Islamist terrorism.

"I condemn in the strongest possible terms the senseless and most heinous acts perpetrated by foreign terrorists," said President Ferdinand Marcos Jr. "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 in Maguindanao del Sur some 200 kilometers (125 miles) away.