Fifty-six people died in a fire at an apartment block in Vietnam's capital Hanoi, police said Wednesday, with witnesses reporting screams from inside and a small boy thrown from the building.

Vietnam's deadliest fire in 20 years started just before midnight on Tuesday (1700 GMT) in the parking floor of the 10-storey building, an area packed with motorbikes, witnesses said.

"56 people have been killed and 37 people injured," the Hanoi police department said in a statement.

At least three children were among the dead, state media reported.

The owner of the apartment block has been arrested on suspicion of violating fire prevention regulations, Hanoi police said.

A woman who lives near the block, who gave her name only as Hoa, said: "I heard a lot of shouts for help. We could not help them much."

"The apartment is so closed with no escape route, impossible for the victims to get out," she told AFP at the site.

Photos by AFP photographers on the scene showed flames and smoke billowing from barred balconies.

The blaze was out by Wednesday morning but rescuers still struggled for hours to get into the building, which is down a narrow alley in a residential area of southwest Hanoi.

The complex's small balconies were surrounded by iron bars, with the apartment block having only a single exit and no emergency ladder on the outside.

Pictures showed a line of charred motorbikes on the ground floor, where the fire began.

Around 150 people lived in the complex, authorities said.

Another witness, Huong, said a small child was thrown from an apartment to escape the flames.

"The smoke was everywhere. There was a little boy thrown from a high floor, I don't know whether he survived or not although people used a mattress to catch him," she said.

We were so scared

Neighbors reported several others jumping from the block.

"There was a middle-aged woman jumping down onto my terrace here. She hit her arm and broke it. Another person also jumped down here and broke their legs," said Dao To Nga.

"A child was put into a plastic basket and lowered down".

Some were able to flee the building using her roof. "This rooftop of my family helped 14-15 people escape," she said.

One survivor, Nguyen Thi Minh Hong, told AFP her family waited in their seventh-floor apartment at the back of the building for five hours before help arrived.

"We were sleeping when suddenly we felt very hot because the power had been cut. My husband opened the door and we heard people shouting for help and we saw smoke," the 34-year-old said.

The family, which included her two children aged six and nine, tried to escape to the roof but were forced back into their apartment due to the heat of the fire.

"I was so scared. We stayed inside the room for five hours… I just tried to calm my kids down by holding a wet towel to their face," she said from Hanoi's Bach Mai Hospital.

"We were between life and death."

Hundreds of people gathered later outside a Hanoi morgue, waiting to hear if their family members and friends were among the victims.

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.

Investigation

Prime Minister Pham Minh Chinh, who visited the site Wednesday as well as survivors in hospital, ordered an investigation into the fire.

Vietnam has experienced several deadly fires in recent years, often at entertainment venues.

A year ago, a karaoke bar blaze near the commercial hub of Ho Chi Minh City killed 32 people and injured 17. The owner was later arrested on charges related to breaching fire prevention regulations.

In the aftermath, the prime minister ordered an inspection of all high-risk venues.

Thirteen people died in 2018 after an apartment complex in Ho Chi Minh City caught fire.

Another 13 died in 2016 in a karaoke venue in Hanoi following a fire.

Southeast Asia frequently sees deadly fires with accidental conflagrations common, a result of lax safety standards.