A series of deadly suicide bomb attacks claimed by Isis on Indonesian churches were carried out by members of a single family, police have said, as the world’s most populous Muslim nation recoiled in horror at the attack on the country’s Christian minority.

The national police chief, Tito Karnavian, said Sunday’s attacks, which killed at least 13 people including the six bombers, and injured more than 40 others, were carried out by children, teenagers and adults from a family who were among 500 Islamic State sympathisers who had returned from Syria.

He said the family’s father detonated car bombs, two sons aged 18 and 16 used motorcycles in their attack and the mother and two daughters, aged 12 and 9, had explosives strapped to them.

“The husband drove the car, an Avanza, that contained explosives and rammed it into the gate in front of that church,” East Java police spokesperson Frans Barung Mangera told reporters at the regional police headquarters in Surabaya. He called on people to remain calm.

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,” he said.

Isis claimed responsibility for the attacks in Surabaya, Indonesia’s second-largest city, the Islamist militant group’s Amaq news agency said, without providing any substantiating evidence to support its claim.

“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.

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 evidence.

Television footage showed the Indonesian Christian Church where the yard in front appeared engulfed in fire. A large blast was heard hours after the attacks, which Mr Mangera said was a bomb disposal squad dealing with a device.

Veiled women had entered the yard where they were stopped by a security guard before an explosion occurred at the same spot, according to the police report.

Television images showed toppled and burnt motorcycles and debris scattered around the entrance of one church and police cordoning off areas as crowds gathered.

At St Mary’s catholic church, the first place of worship to be attacked, the bombing was perpetrated while the church prepared to hold another service, after an earlier one had concluded.

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

The near-simultaneous attacks took place during Sunday morning masses in the predominantly Muslim country, days after police ended a riot at a detention centre following a 36-hour standoff that left five dead and five injured after hostages were taken.

On Friday, a group of 70 prominent Muslim scholars meeting in Bogor, Indonesia, made a joint declaration denouncing violent extremism and terrorism, including suicide attacks, which they categorically stated are against Islamic principles.

Indonesia has seen a recent resurgence in homegrown militancy, although it has carried out a sustained crackdown since bombings by al-Qaeda affiliated radicals in Bali in 2002 killed 202 people.

Churches have also been targeted previously, including near-simultaneous attacks on churches there at Christmas in 2000 that killed about 20 people.