MANILA -- Philippine President Ferdinand Marcos Jr condemned a deadly bombing on Sunday, blaming "foreign terrorists", as police and the military strengthened security in the country's south and around the capital Manila.

At least four people were killed and at least 50 injured after a bomb exploded during a morning Catholic Mass in a university gymnasium in Marawi, a city in the south of the country besieged by Islamist militants for five months in 2017.

"I condemn in the strongest possible terms the senseless and most heinous acts perpetrated by foreign terrorists," Marcos said in a statement. "Extremists who wield violence against the innocent will always be regarded as enemies to our society."

Law enforcement operations to bring to justice the perpetrators of the "terrorist activity" will "continue unabated", Defense Secretary Gilberto Teodoro told a press conference.

There were "strong indications of a foreign element" in the bombing, Teodoro said, refusing to elaborate so as not to compromise ongoing investigation.

Fragments of a 16-mm mortar were recovered at the scene, senior police official Emmanuel Peralta told the press conference.

High alert

The blast in Marawi, capital of Lanao del Sur province, followed a series of military operations against local pro-Islamic State groups in the southern Philippines, the military chief said, including one on Sunday in Lanao del Sur that led to the killing of a leader of the Dawlah Islamiya-Maute group.

"It is possible that what happened this morning was a retaliatory attack," Armed Forces Chief Romeo Brawner told the press conference.

The Islamic State-linked Maute seized Marawi on May 2017, seeking to make it a Southeast Asian "wilayat" – or governorate - for Islamic State. In the ensuing five-month battle, Islamist fighters and Philippine forces killed more than a thousand people, including civilians.

Military officials surveyed the gym at the Mindanao State University, which appeared intact except for burn marks in the centre where the explosion occurred, according to images shared by the Lanao del Sur government on Facebook. 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.

Police offices in Mindanao and the capital region were placed on high alert and police checkpoints tightened "to prevent possible follow-up incidents", police official Peralta said.

The coast guard 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," the school posted 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.