DHAKA - Two explosions ripped through a crowd Saturday, killing six people and injuring some 50 in Bangladesh's northeastern city of Sylhet as army commandos stormed extremists hideout, police said.

The "powerful" blasts went off some 400 yards from the hideout, targeting police and hundreds of onlookers who were watching the commandos conduct an anti-militant operation at a five-storey apartment building, police said.

Deputy Commissioner of Sylhet police Basudeb Banik told AFP "six people including two police officers were killed" in the attack, which was claimed by the Islamic State (IS) jihadist group via its propaganda agency Amaq.

He said around 50 people were injured including about a dozen police and security officers.

Several people were reported to be in critical condition, including the head of intelligence of the elite Rapid Action Battalion (RAB), which is tasked with combating extremism in the country, he said.

Police primarily suspect the blasts were the work of a new faction of the homegrown extremist group, Jamayetul Mujahideen Bangladesh (JMB), which has been blamed for a wave of attacks in recent years.

Police could not confirm if it was a suicide attack but believe one of the attackers could be among the dead.

IS claimed via Amaq that there were "dozens killed and wounded among Bangladeshi forces as a result of the detonation of an explosive device", according to US-based monitoring agency SITE Intelligence Group.

Banik rejected the claim, saying IS does not have any presence in the country.

The blasts occurred after an hours-long exchange of gunfire and commandos rescued "78 civilian hostages" from the hideout where several militants were holed up in a ground-floor apartment.

"Our main task was to rescue the hostages, which we have done successfully. We were able to rescue all 78 people safely," army spokesman Brigadier General Fakhrul Ahsan told reporters.

He said the extremists were still inside the apartment building where they had barricaded themselves and laid explosive devices. "As a result the whole operation is being conducted carefully," he said.

The commandos backed by armoured personnel carriers launched the operation after a more than 30-hour standoff that began early Friday morning when police sealed off the building as militants detonated small bombs.

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 extremists," police spokesman Musa said, adding they shouted "Allahu Akbar" (God is the greatest).

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 extremists this month including one at a police checkpoint near the country's main international airport on Friday night.

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

This month a police elite unit also stormed a building outside the port city of Chittagong, killing four members of JMB, including a woman. IS has claimed responsibility for a wave of killings since 2015 including 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 the attacks.

COMMANDOS KILL TWO MILITANTS

Bangladesh army commandos shot dead two suspected extremists at a hideout in the northeast Sunday, a day after six people died in explosions claimed by the Islamic State group. "Our commandos could effectively engage two of them. We can confirm the two were killed," Brigadier General Fakhrul Ahsan told reporters in the city of Sylhet.

Ahsan said the commandos, armed with rocket launchers as well as rifles, located the men wearing suicide vests on the ground floor of a five-storey building and shot them dead.  "The suicide vest of one of them (who died) also exploded," he said.

Ahsan said the operation was continuing because one or more of the extremists - who were well armed and equipped - were still holed up in the building where they had laid improvised explosive devices.

He did not comment on their identities but police said they were members of a homegrown militant outfit blamed for a wave of attacks in recent years.