Bangladesh firefighters said Friday that glaring safety lapses were responsible for a Dhaka restaurant blaze that killed 46 people, with more deaths likely among those rushed to hospital in critical condition.

Thursday night's fire began at a popular biryani restaurant at the bottom of a seven-floor commercial property in the capital's upscale Bailey Road neighbourhood.

The entire building, home to several other eateries, was soon engulfed by flames that took fire crews two hours to bring under control.

Fire service operations director Rezaul Karim told AFP the blaze had been made worse by numerous cooking gas cylinders stored haphazardly in stairwells and restaurant kitchens.

"People heard the explosions of several gas cylinders during the fire," he said.

Main Uddin, the national fire services chief, said the building lacked safety measures.

"It did not have at least two staircases or a fire exit," he told AFP. "Most of the people died from suffocation."

Fire officials earlier told reporters they suspected the inferno began when one of the gas cylinders accidentally caught fire.

Police inspector Bacchu Mia told AFP that two more people had succumbed to their wounds on Friday while being treated in hospital.

"The death toll is now 46. Two people have died from injuries -- one at the Dhaka Medical College Hospital and another at the Police Hospital," he told AFP.

Around 15 people remained in critical condition, he added.

- 'Turned into charcoal' -

At a hospital treating the wounded, 30-year-old Asif Pathan told AFP that his cousin Minhaj Khan had been dining at the restaurant when the fire broke out and was killed.

"His friend escaped by jumping through the window, but Minhaj couldn't," Pathan said. "His body has turned into charcoal."

Pathan said he was waiting for the hospital to conduct DNA tests to confirm the identity of his cousin's body before it was released to his family.

Members of the public helped fire crews carry hoses and rescue survivors who clambered down the outside walls to safety as firefighters fought to bring the blaze under control.

"We were at the sixth floor when we first saw smoke racing through the staircase. A lot of people rushed upstairs," Sohel, a restaurant manager who gave only his first name, told AFP.

"We used a water pipe to climb down the building. Some of us were injured as they jumped."

At one point at least 50 people were on the rooftop waiting to be rescued by fire cranes, Kamruzzaman Majumdar, an environmental science professor who was among the stranded, wrote in a Facebook post.

Police investigators were seen walking inside the gutted building and documenting the wreckage on Friday morning, hours after the government ordered an investigation into the fire's origins.

Hundreds of anxious family members rushed to the nearby Dhaka Medical College Hospital overnight as ambulances brought the dead and injured to the clinic.

Explosions and fires are frequent in buildings and factories across Bangladesh, where safety standards are lax and corruption often allows them to be ignored.

Deadly blazes are typically sparked by gas cylinders, faulty air conditioners and bad electrical wiring.

Bangladesh's worst fire took place in 2012, when a blaze ripped through a garment factory on Dhaka's outskirts, killing at least 111 people and injuring more than 200 others.