Knife-wielding attacker wounds four church-goers in Indonesia: Police

Police said they were investigating possible motives for the attack by the suspect, believed to be a university student, at a Catholic church service in Yogyakarta, the gateway to the cultural centre of Indonesia's main island of Java.

"We cannot confirm yet if this was a terror-related incident," said Yogyakarta police spokesman Yuliyanto, who goes by one name like many Indonesians. "We can confirm the suspect has been detained and is being treated at hospital."

Yuliyanto said the unidentified attacker had been shot in the stomach by police. The condition of the wounded church-goers was not immediately clear.

Indonesia is an officially secular country and has the world's largest population of Muslims, as well as sizeable minorities of Christians, Hindus, and those who adhere to traditional faiths.

Communal and religious tensions have been on the rise in recent years as calls by hardline groups for sharia, or Islamic law, to be implemented nationally grow louder.

Indonesia has also seen a resurgence in homegrown radicalism, inspired in part by the militant Islamic State group.