Dhaka: Bangladesh commandos Saturday stormed a hideout of suspected Islamist extremists in the country's northeast, rescuing at least 78 civilians, in a major anti-militant drive.

The commandos backed by armoured personnel carriers launched the operation after a more than 30-hour standoff with the extremists who holed up in a five-storey building in the city Sylhet. 

"All 78 people from 28 civilian families who were trapped in the building for more than a day have been rescued by the army's para commando battalion," armed forces spokesman Colonel Rashidul Hasan told AFP.

"The operation began at 7:00 am (0100 GMT) in the morning and is still going on," he said, adding the commandos "exchanged fire" with the suspected militants who were confined in a ground-floor apartment of the building.

The spokesman could not say how many extremists were in the building, but police said there were at least two including a woman.

"They are Islamist extremists," Sylhet police spokesman Zedan Al Musa, adding they shouted "Allahu Akbar" (God is the greatest).

Acting on a tip police raided the building at 2:00 am Friday and cordoned off the whole area after suspected militants detonated small bombs, police said.

Police used loudspeakers to ask the extremists to surrender, but they refused to give up, Musa said.

The raid came after a series of suicide attacks on security camps by Islamist extremists this month including one at a police check-point near the country's main international airport on Friday night.

Two of the attacks including Friday's blast in which the suicide attacker was killed were claimed by Islamic State group.

This month a police elite unit also stormed a building outside the port city of Chittagong, killing four members of the homegrown extremist group Jamayetul Mujahideen Bangladesh (JMB) including one woman.

A forensic doctor confirmed that Friday's attack was a suicide blast with the bomber apparently detonating a bomb tied to his body.

The attack was the third since last Friday, when a suicide bomber blew himself up at a camp for the country's elite security force near the airport.

IS has also claimed responsibility for a wave of killings since 2015 including for a major attack on a Dhaka cafe last year in which 22 people, including 18 foreign hostages, were killed. 

The Bangladeshi government denies IS has any presence in the country, arguing instead that a new faction of JMB was behind that and other attacks.