The Islamic State (ISIS) has claimed responsibility for what it said was a targeted bombing of Christians in the Philippines that killed four people and injured more than 50.

The bombing on Sunday happened as worshippers gathered for a Catholic church service in the gymnasium at Mindanao State University in Marawi City, Philippines, according to news reports. ISIS said it was targeting “a large gathering of Christian disbelievers.” The attack coincided with the launch of the Christmas advent season, prompting the Philippine government to place security forces on high alert.

Marawi City, which has a large Muslim population, was terrorized by ISIS militants for five months in 2017 until Philippine forces, with help from the United States and Australia, regained control. But not before ISIS murdered more than 1,100 people and displaced many others.

“I condemn in the strongest possible terms the senseless and most heinous acts perpetrated by foreign terrorists upon the Mindanao State University,” Philippine President Ferdinand Marcos Jr. said. “Extremists who wield violence against the innocent will always be regarded as enemies to our society.”

Meanwhile, the U.S. State Department condemned “in the strongest terms the horrific terrorist attack. … We mourn those killed in the attack, and our thoughts are with the injured. 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.”

According to news reports, Philippine officials said the attack might have been retaliation for Philippine military action on Friday that killed 11 suspected terrorists in the southern part of the country, which has the heaviest Islamic presence.

“We are looking at possible angles,” Philippine military chief of staff Gen. Romeo Brawner said, according to the Los Angeles Times.

The bombing comes against the backdrop of heightened terror threats across the globe after Iran-backed Hamas attacked Israel on Oct. 7, erupting into war.

In the U.S., threats by pro-Hamas radicals have been reported against Christians and Christian churches, and several Christmas events in the U.S. have been disrupted by pro-Hamas protesters. At one tree lighting in New York City, protestors who attempted to shut down the event held pro-Hamas signs, including one that asked, “How Many Churches Need to be Bombed to Make Christians Care?”

“The fact that these mobs are determined to undermine a pivotal holiday such as the birth of Christ reveals the harsh reality of Muslim extremists,” Christian apologist and author Alex McFarland wrote. “In the face of such religious aggression, we as Christians must remain firm in our faith while still expressing the love of Christ. These attacks must renew our resolve to celebrate the birth of our Lord and Savior, even in the face of such violent persecution—for we were not promised an easy road, but rather the presence of the Spirit in the face of difficulties.”

Photo courtesy Provincial Government of Lanao Del Sur