A family of six launched suicide attacks on Christians attending services at three churches in Indonesia's second-largest city of Surabaya, killing at least 13 people and wounding 40, officials said.
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.
They blamed the bombings on the Islamic State-inspired group Jemaah Ansharut Daulah (JAD).
JAD is an umbrella organisation on a US State Department "terrorist" list that is estimated to have drawn hundreds of Islamic State sympathisers in Indonesia.
Islamic State 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 the attacks had killed at least 13 people and 40 had been taken to hospital, including two police officers. He called on people to remain calm.
Streets around the bombed churches were blocked by checkpoints and heavily armed police stood guard as forensic and bomb squad officers combined the area for clues.
Television footage showed one church where the yard in front appeared engulfed in fire, with thick, black smoke billowing up. A large blast was heard hours after the attacks, which Mangera said was a bomb disposal squad dealing with a device.
The attacks come days after militant Islamist prisoners killed five members of an elite counter-terrorism force during a 36-hour standoff at a high security jail on the outskirts of the capital, Jakarta.
The church attacks were likely linked to the prison hostage standoff, said Wawan Purwanto, communication director at Indonesia's intelligence agency.
"The main target is still security authorities, but we can say that there are alternative (targets) if the main targets are blocked," he said.
Australian Associated Press