Philippines security forces killed Thursday a leader of a militant organization affiliated with the Islamic State group, also known as ISIS, according to local authorities, who said they anticipated reprisal attacks.

Police reportedly discovered Mohammad Jaafar Maguid, leader of the ISIS-linked Ansar al-Khilafah Philippines (AKP), at a beach resort in the southern province of Sarangani after citizens tipped authorities to the presence of armed men. Security forces clashed with Maguid, who they said resisted arrest and was killed in the fight around midnight. Three other AKP militants were arrested and weapons recovered during the operation, which police chief Ronald Dela Rosa hailed as a victory over terrorism.

"I strongly believe that we have effectively broken the backbone of the militant Ansar Al-Khilafah Philippines," Rosa said at a press conference. "He is the recognized ISIS leader in that area," he added. 

Jaafar, who went by the name "Tokboy" and whom police considered "the most wanted man in the area" for his group's involvement in a number of attacks and kidnappings, was the latest casualty in the President Rodrigo Duterte's efforts to tackle insurgency in the Philippines that has killed at least 120,000 people over four decades. Numerous militant groups, including Abu Sayyaf and Maute, have pledged their allegiance to ISIS and operate openly in the country's restive southern provinces.

Duterte told local media Wednesday that ISIS militants were behind a recent series of bombings that killed 14 people and injured dozens. The leader, whose controversial remarks have gained him media attention in the past, claimed ISIS influence was so strong in the Philippines that he had cousins who had joined the militant organization. That same day, militants staged the largest jailbreak in the nation's history, freeing over 150 rebel prisoners and clashing with security forces.

Following Thursday's police operation, authorities warned that the group could retaliate and announced heightened security for the upcoming Feast of the Black Nazarene or Traslacion, a religious celebration expected to draw as many as 18 million people in Manila.