Philippine security forces killed the leader of a militant group supporting Islamic State in a clash early on Thursday, the country’s police chief said, warning against possible retaliation.

President Rodrigo Duterte recently cautioned against Islamic State taking root in the southeast Asian country, saying it needed to avoid “contamination”.

“I strongly believe that we have effectively broken the backbone of the militant Ansar Al-Khilafah Philippines (AKP),” Ronald Dela Rosa told a news conference to announce the death of the group’s leader, Mohammad Jaafar Maguid.

Three AKP colleagues of Maguid were also arrested in a police operation shortly after midnight at a resort in the southern province of Sarangani, he added.

“He is the recognized ISIS leader in that area,” Dela Rosa said, adding that Maguid, who goes by the alias “Tokboy”, was the “most wanted person” in the country’s south, sought for his involvement in crimes ranging from arson and murder to bombings.

Police recovered two armalite rifles, a hand grenade, and M-16 magazines, from Maguid and the three men, identified as Matahata Dialawe Arboleda, Ismael Sahak, and Morhaban Veloso.

The AKP and the Maute militant group, which has also pledged allegiance to Islamic State, are among a handful of small groups authorities blame for years of unrest in the Philippine south.

The killing and the arrests would  “momentarily weaken” the group, Dela Rosa said, but a new leader could emerge eventually and launch fresh attacks.

“We expect some retaliation from the ISIS-inspired groups,” he added. “We should expect, and we should be very vigilant against, this possible retaliation.”

Philippine security forces are on full alert ahead of Monday’s feast of the Black Nazarene in the capital Manila. It  usually draws millions of devotees for a procession through the streets that runs for hours.