The 18-year-old charged at the priest with an ax during Sunday mass in the city of Medan, the capital of North Sumatra province, according to Indonesian police.

The assailant reportedly only caused a slight injury to the priest's hand, national police spokesman Rafli Amar said. The youth also attempted to detonate an explosive device in his backpack, but it only burned without exploding, Amar added.

A witness, Markus Harianto Manullan, said he and the attacker sat in the same row in the crowded church.

"I saw him fiddling with something in his jacket, and then I heard a small explosion and he immediately ran to the podium," Manullan said.

The teenager used his backpack to smuggle the ax and the bomb into the building.

Members of the congregation restrained the attacker and called police. The assailant told authorities he was not working alone, police spokesman Amar said. While the attacker's motive was not immediately clear, he carried a symbol indicating support for the so-called "Islamic State" (IS) group.

With a population of over 250 million people, Indonesia is the largest Muslim-majority country in the world. Various extremist groups have targeted religious and other minorities in the nation during recent years.

A bomber linked to IS blew himself up outside a police station in Central Java in July.

dj/jlw (AP, dpa, AFP)