Islamic State-allied militants in the Philippines have taken a Catholic priest and a group of church-goers hostage, threatening to kill them if the nation's military does not cease its current offensive against them.

The hostages were taken during a militant siege in the southern Philippines city of Marawi on Tuesday and Wednesday. Militants also burned the Catholic cathedral of Marawi.

Archbishop Socrates Villegas, president of the Catholic Bishops' Conference of the Philippines, denounced the hostage-taking. He said the priest and the hostages had no involvement in the conflict between the military and the militants.

"He was not a combatant. He was not bearing arms. He was a threat to none," the archbishop said. "His capture and that of his companions violates every norm of civilized conflict."

The country's Catholic bishops have urged prayers for the captured priest and the other hostages in the area. While the majority of the Philippines is Catholic, they make up only a small percentage of the population in Marawi, a mostly Muslim city of about 200,000 people, located on the island of Mindanao.

About 100 armed militants moved through Marawi on Tuesday, the Associated Press reports. They beheaded a police chief and burned buildings, including the bishop's residence. They raised the black flag of the Islamic State group while also taking the hostages.

Responsible for the attack is the Maute group, a clan-based group with members in Marawi. It is one of under a dozen new armed Muslim groups that have pledged allegiance to ISIS. The groups have formed a loose alliance, reportedly led by Isnilon Hapilon, a commander of the Abu Sayyaf militant group.

The militants' siege of Marawi followed an army raid on the hideout of Hapilon. The militant leader has pledged allegiance to ISIS and the United States has offered a $5 million reward for information leading to his capture.

Bishop Edwin de la Peña of Marawi was not at home at the time of the attack, but his secretary is reportedly among the hostages. He received a phone call from a militant who used his secretary's phone. The militant introduced himself as a member of ISIS and demanded a unilateral ceasefire.

"They want a ceasefire and for the military to give them access out of Marawi. Otherwise, they will kill the hostages," Bishop de la Peña told CBCP News.

The bishop reported that he was allowed to speak with Fr. Chito Suganob, the captive priest who is the vicar general of the Territorial Prelature of Marawi, in order to make their demands clear.

In addition to the priest, hostages include three church staffers and ten worshipers, the Associated Press said.

Bishop de la Peña himself barely missed being taken hostage.

"I was supposed to go to Marawi yesterday but I was asked to cancel my trip because of the siege," he said.

Archbishop Villegas, the Catholic bishops' conference president, urged prayers for peace and asked the militants to show mercy.

"We call on the Maute group that claims to bear arms in the name of a Merciful and Benevolent God – the very same God we Christians worship and adore – to do the One God true honor by the mercy and benevolence that are two of our God's most exalted attributes," he said.

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The archbishop also addressed the response of government forces, saying, "We beg of them to make the safety of the hostages a primordial consideration."

President Rodrigo Duterte, who has been heavily criticized for a brutal crackdown on illegal drugs, has cut short his trip to Russia and placed all of Mindanao island under martial law. The president has sought peace talks with two large Muslim rebel groups in the country's south but has ordered the military to destroy smaller extremist groups like the Maute.

"It is difficult to root out because they are from there," political analyst Ramon Casiple told the Associated Press. "The Mautes are embedded in the population."

The group was blamed for a September 2016 bombing that killed 15 people in southern Davao city, the president's hometown. A military raid on their jungle camp last month reportedly found homemade bombs, grenades, combat uniforms, and passports of suspected Indonesian militants.