IS-inspired family attacks Java churches

A family of IS sympathisers, parents and four children, the youngest just 9, turned suicide bombers and attacked three Indonesian churches, killing 13 people.

A family of six killed 13 people and wounded 40 in suicide attacks on three churches in Indonesia. (AAP)

A family of six launched suicide attacks on Christians at three churches in Indonesia's second-largest city of Surabaya, killing at least 13 people and wounding 40.

Indonesia, the world's largest Muslim-majority country, has seen a recent resurgence in homegrown militancy and police said the family who carried out Sunday's attacks were among 500 Islamic State sympathisers who had returned from Syria.

"The husband drove the car, an Avanza, that contained explosives and rammed it into the gate in front of that church," East Java police spokesman Frans Barung Mangera told reporters at the regional police headquarters in Surabaya.

The wife and two daughters were involved in an attack on a second church and at the third church, "two other children rode the motorbike and had the bomb across their laps", Mangera said.

The two daughters were aged 12 and 9 while the other two, thought to be the man's sons, were 18 and 16, police said.

Police blamed the bombings on the Islamic State-inspired group Jemaah Ansharut Daulah.

IS claimed responsibility for the attacks in a message carried on its Amaq news agency.

"This act is barbaric and beyond the limits of humanity, causing victims among members of society, the police and even innocent children," President Joko Widodo said during a visit to the scene of the attacks.

East Java police spokesman Mangera said at least 13 people had been killed and 40 had been taken to hospital, including two police officers. A large blast was heard hours after the attacks, which Mangera said was a bomb disposal squad dealing with a device.

Streets around the bombed churches were blocked by checkpoints and heavily armed police stood guard as forensic and bomb squad officers combed the area for clues.

At St Mary's Catholic church, the first place of worship to be attacked, the bombing happened after an earlier mass was over and when the church was getting ready to hold another service.

Separately, an internal police report reviewed by Reuters said a suspected bomb exploded in a car in the parking lot of a Pentacostal church, setting alight dozens of motorbikes.

In the third location, the Indonesian Christian Church, veiled women entered the church's yard where they were stopped by a security guard before an explosion occurred at the same spot, according to the police report.