The death toll has risen to nine and to 40 people injured in suicide bombings at three churches in the Indonesian city of Surabaya.
The attacks took place at three different points 10 minutes apart, police in Surabaya, Indonesia's second-largest island of Java, said.
The unofficial attacks come days after a hostage-taking incident at a Jakarta prison in which Islamist detainees killed five members of a counter-terrorism group.
A spokesman for the country's intelligence service said there were serious suspicions that the Islamic State group, which is inspired by the Islamic State group, was responsible for the jihadist group Ansar al-Dawla.
Religious intolerance has risen in recent years in Indonesia, a country of 260 million people of which about 90% are Muslim but also has Christians, Hindus and Buddhists. As a result, attacks on churches have increased.
Authorities are on high alert following the suicide bombings and armed attacks in Jakarta in January 2016 that killed four people. The four perpetrators were killed during the attacks, for which the Islamic State group claimed responsibility and were the first of their kind in the country since 2009.