ISIS, the Islamic State group, has claimed responsibility for a deadly explosion that ripped through a Catholic mass service at a university gym in the southern Philippines on Nov. 4.

At least four people were killed, and dozens of others were injured in the blast at the Mindanao State University in Marawi City, according to authorities cited by CNN.

In a communique, ISIS said its fighters "detonated an explosive device on a large gathering of Christian disbelievers in Marawi City," according to the SITE Intelligence Group, a counterterrorism threat intelligence organization that tracks the online activity of extremist groups.

The attack was carried out in a university gymnasium in Marawi, a city in the south of the country besieged by Islamist militants for five months in 2017, Reuters reported.

The Islamic State group, which wields influence in the country's south, said on Telegram its members had detonated the bomb.

Earlier, before Islamic State's claim, Philippine President Ferdinand Marcos Jr condemned "the senseless and most heinous acts perpetrated by foreign terrorists."

In Rome, Pope Francis offered prayers for the victims during his regular Sunday address, and, in a separate written message, he appealed to "Christ the prince of peace, grant to all the strength to turn from violence and overcome every evil with good".

Police and the military strengthened security in the country's south and around the capital Manila.

The United States condemned the "horrific terrorist attack" in a government statement.

"The United States is in close contact with our Philippine partners and stands with the people of the Philippines in rejecting this act of violence," State Department Spokesperson Matthew Miller said.

     - ARMY CHIEF 

Army chief General Romeo Brawner said previously that the attack may have been revenge for recent military operations against three militant groups -- Dawlah Islamiyah-Philippines, Abu Sayyaf and Maute, AFP reported

One of those operations killed an Abu Sayyaf "bomb expert", who was allegedly responsible for the 2019 deadly attack on a cathedral on the southern island of Jolo, Brawner told CNN Philippines on Tuesday.

Pro-Islamic State Maute and Abu Sayyaf militants -- including foreign and local fighters -- held Marawi under siege in 2017.

The Philippine military seized back the ruined city after a five-month battle that claimed more than 1,000 lives.

Brawner refuted charges there was an intelligence failure ahead of the Nov. 4 bombing.

He said there should have been "more security measures in place" after the military warned local security forces, government officials and the public of possible retaliatory attacks.

More than 85 percent of Philippines' population of 116 million are Christians and 6.4 percent are Muslims and Mindanau has a significant Islamic population.