According to local police, a single family was behind the attack, which has been deemed as the deadliest in more than a decade's time.

The family consisted of six people, including a 9-year-old girl.

According to reports, the mother and her two daughters were involved in a suicide attack at a church, while the father and his two sons attacked two more churches.

Local officials have said that the family may have spent some time with the Islamic State in Syria and were related to a local IS-inspired network, Jemaah Ansharut Daulah (JAD).

The officials added that the father drove a bomb filled car into the premises of Surabaya Centre Pentecostal Church.

The mother and the daughters, who had bombs strapped to them, attacked the Diponegoro Indonesian Christian Church.

The daughters were aged 9 and 12, while the sons were 16 and 18 years old, respectively.

They reportedly rode motorcycles while attacking Santa Maria Catholic Church. 

Country's President Joko Widodo visited Bhayangkara Hospital to meet victims of the attacks.

"The president made the visit after reviewing two bombing incident locations namely GKI church on Jalan Diponegoro Surabaya and Central Pentecost Church Surabaya on Jalan Arjuna," reported country's Antara news agency.

He has appealed people in the country to remain calm.

"I appeal to all people across the country to remain calm, maintain unity and alertness," he was quoted as saying by the agency.

Indonesia's Foreign Minister Retno Marsudi has condemned the attack.

"Strongly condemn the terrorist attack in Surabaya - Ret We will not back down in the fight against terrorism Let us unite in the fight against terrorism #UniteAgainstTerrorism #WeAreNotAfraid," she tweeted.

The US Embassy in Indonesia also condemned the attacks on the churches.

"The United States strongly condemns this morning’s attacks on three churches in Surabaya. These attacks on peaceful worshipers are an affront to the tolerance and diversity embraced by Indonesians.  The United States stands with the people of Indonesia, and we offer our deepest condolences to the families of the victims," US Embassy said in a statement.