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

The "powerful" blasts went off some 400m 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.

Sylhet police's Deputy Commissioner Basudeb Banik said "six people including two police officers were killed" in the attack, which was claimed by the Islamic State in Iraq and Syria (ISIS) 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 added.

Police primarily suspect the blasts were the work of a new faction of the home-grown extremist group, Jamayetul Mujahideen Bangladesh, 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. ISIS 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.

Mr Banik rejected the claim, saying ISIS 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," the army spokesman, Brigadier-General Fakhrul Ahsan, told reporters.

AGENCE FRANCE-PRESSE