Faulty construction work could be the cause of the collapse

Police said the incident happened early on Wednesday morning in the densely populated old part of Dhaka when residents were sleeping.

A rescue operation is underway with the military helping police and firemen.

The prime minister has ordered old and risky structures to be identified to prevent future building collapses.

Narrow alleys

The BBC's Waliur Rahman in Dhaka says that residents awoke to hear the noise of falling masonry and came on to the streets to see the tall building block crumble.

Police officer Giasuddin Ahmed told the BBC from the scene that the rescue operation could take several hours because crane and other clearing equipment could not reach the site due to the narrow alleys.

I was awoken by a big bang to find the top three floors of the building had tilted to one side Building collapse survivor

"The lane in front of the collapsed building is so narrow that rescue vehicles are unable to move," he said.

Police are still not sure how the building collapsed, but they suspect poor foundation and construction work could be contributory factors. At least four people are still thought to be trapped in the rubble.

The collapse happened early in the morning when residents were asleep.

"We don't know whether the owner had the permission to raise another three storeys on top of the existing two-storey building," Mr Ahmed said.

"We will investigate in that line also," he said.

Locals say the building in the crowded business district was home to several families and contained some shops.

Most of the buildings in the area are over 100 years old and were declared unsuitable for human habitation by the city authorities several years ago.

But town planners say this warning was ignored.

The Prime Minister, Khaleda Zia, has now ordered officials "to do everything possible" in the rescue operation.

She says that she wants to put a stop to the construction of risky structures in the capital and has asked officials to take measures to stop such incidents from happening again.

Mrs Zia appealed to owners and residents of such buildings to co-operate with the authorities to make residencies safer.