At least 11 people have been killed and 40 injured in a wave of coordinated bomb attacks at three churches in the Indonesian port city of Surabaya, at least one of them perpetrated by a suicide bomber.

The State Intelligence Agency said Islamic State-linked group Jemaat Ansharud Daulah (JAD) was behind the attacks on Sunday.

“I was sitting at the canteen in front of the church when I heard a loud explosion,” Endang, a worshipper at Santa Maria told Kompas TV.

“There were a lot of people at that time. I saw dead bodies and people injured,” she said.

Video surveillance footage is reported to have captured a lone man on a motorcycle riding into the church, followed seconds later by a blast and hail of debris.

BREAKING: Three churches suicide bombed in Indonesia. At least 2 people were killed in the attacks.

The war on Christianity continues. https://t.co/bco8T1uKBO

— 🏂💨 (@JimHill92) May 13, 2018

Indonesian police had been on alert after intelligence reports indicating Islamist militants were planning a major terror offensive.

“They had planned to attack police targets on May 11, but because the police were prepared, they picked alternative targets,” National Intelligence Agency spokesman Wawan Purwanto told Metro TV.

Medics attend to this worshipper’s wounds outside one of the bombed churches.

The world’s largest Muslim-majority country, Indonesia has witnessed a recent increase in homegrown militancy, with police routinely stationed at churches to guard against terror strikes.

The latest attacks come as Aman Abdurrahman, an alleged leader of JAD, is on trial for a 2016 attack in Jakarta that killed eight people, including four militants.

East Java police spokesman Frans Barung Mangara said the first explosion, a suspected suicide bombing, occurred during Sunday Mass at the Santa Maria Catholic church in a suspected suicide bombing at around 7.30am.

It was followed over the next 10 minutes by blasts at the Pentecostal and  Protestant churches.

Television footage showed burning vehicles outside the Pentecostal church and police trying to defuse what appeared to be an unexploded device outside a third church.

Surabaya, the capital of East Java province, is Indonesia’s second-largest city.

The wave of bombings follows a riot by convicted terrorists who captured and executed five police officers and guards at a maximum-security prison on the outskirts of Jakarta.

-with wires