At least 56 people are known to have died and dozens were injured in a huge fire in an apartment block in Vietnam's capital Hanoi.

Some 37 people were injured in the fire, police said, adding that 39 victims have been identified.

The fire, which broke out during the night, has been extinguished. The owner of the building has been arrested.

Authorities in fast-growing Hanoi say many newly-built apartments do not meet fire safety regulations.

The city's population has quadrupled to 5.25 million in the past 20 years.

The cause of the fire is being investigated but witnesses said it started in the parking floor of the building, which was packed with motorbikes.

Residents described hearing a loud bang at around 23:00 local time on Tuesday (16:00 GMT), and then seeing black smoke rising through the building.

One family said they had to escape by smashing the metal railings blocking their window, and putting a ladder across to a neighbouring building.

"I heard a lot of shouts for help. We could not help them much," Hoa, a woman who lives nearby, told the AFP news agency.

"The apartment is so closed with no escape route, impossible for the victims to get out."

Another witness saw a little boy thrown from a high floor to help him escape the flames, AFP reports.

"The smoke was everywhere. I don't know whether he survived or not although people used a mattress to catch him," she said.

Fifteen fire engines were sent to assist but could not get close to the burning apartment block because the alley it was in was too narrow.

Hundreds have gathered outside a morgue in the west of the city to hear if their loved ones died in the blaze.

"I lost my daughter, who was staying with her mother," one man told AFP.

Unsure where his wife was, he said: "I guess she did not make it either."

One group of five women, sitting on the floor outside the morgue, said their "whole family had gone".

"They were our children and grandchildren," they said.

Police have detained the owner of the building, Nghiem Quang Minh, accusing him of violating fire-prevention regulations, the government said in a statement, adding that an investigation is under way.

The blaze highlights the challenges of managing fire safety in the region's fast-growing and poorly regulated cities.

A year ago, 33 people died in a fire at a karaoke club in southern Vietnam where windows were bricked up, blocking escape.

There have been many similar tragedies in other South East Asian countries like Thailand, where regulations were found afterwards either to be inadequate, or in many cases simply not enforced.

Additional reporting by Simon Fraser and George Wright in London