The inferno -- the financial hub's worst in decades -- took hold on Wednesday afternoon in an eight-building housing estate with 2,000 apartments and sent shockwaves through the city, which has some of the world's most densely populated and tallest residential blocks.

Flames were still spotted in some of the windows while multiple fire hoses sprayed the scorched exteriors, an AFP reporter saw around noon on Thursday.

Crowds gathered in nearby streets and public areas to organise aid for displaced residents and firefighters, part of a spontaneous effort that drew people from across the city.

"It's truly touching. The spirit of Hong Kong people is that when one is in trouble, everyone lends support... It shows that Hong Kong people are full of love," said Stone Ngai, 38, one of the organisers of an impromptu aid station.

Police said early on Thursday they arrested three men in connection with the fire, after flammable materials left during maintenance work caused the blaze "to spread rapidly beyond control".

Officers searched premises in a separate neighbourhood on Thursday morning, taking away documents in binders in relation to the three people arrested, who police suspect "acted with gross negligence" by leaving foam packaging at the site of the fire.

Hong Kong authorities will immediately inspect all housing estates undergoing major works following the disaster, the city's leader John Lee said.

- Banging on doors -

Multiple residents of Wang Fuk Court, located in Hong Kong's northern district of Tai Po, told AFP that they did not hear any fire alarm and had to go door-to-door to alert neighbours to the danger.

"The fire spread so quickly. I saw one hose trying to save several buildings, and I felt it was far too slow," said a man surnamed Suen, recalling his plight the day before.

"Ringing doorbells, knocking on doors, alerting the neighbours, telling them to leave -- that's what the situation was like."

Another displaced resident, Wong Sik-kam, recalled how his son was one of the firefighters dispatched to the scene.

"My son called me and told me about the fire... I thought it was just a normal fire, like a kitchen accident that would be put out. Who knew it would get so bad?" Wong said.

Hong Kong's fire department raised the death toll to 44 on Thursday morning.

Among the dead was a 37-year-old firefighter, who was found with burns on his face half an hour after losing contact with colleagues, according to the fire service director Andy Yeung.

A government spokesman told AFP that 61 people were being treated in hospital. Fifteen were in a critical condition, 27 in a serious condition and 19 were stable.

City leader Lee said in the early hours of Thursday that 279 were unaccounted for, though firefighters said later that they had established contact with some of those people.

Lee said more than 900 people sought refuge at temporary shelters overnight.

The Indonesian consulate said around noon that two of the deceased were Indonesians working as migrant domestic workers.

- 'Unable to reach people' -

On Wednesday night, sections of charred scaffolding fell from the burning blocks and flames could be seen inside apartments, sometimes belching out through windows into the night sky, casting an eerie orange glow on surrounding buildings.

"The temperature at the scene is very high and there are some floors where we have been unable to reach people who requested help, but we will keep trying," said Derek Armstrong Chan, the deputy director of fire service operations.

He said the wind and drifting debris likely spread the fire from one building to another, although he added that authorities are investigating the cause of the blaze.

Chinese President Xi Jinping expressed condolences to the victims, including "the firefighter who died in the line of duty", according to state media.

Lee said he was "deeply saddened" and that all government departments were assisting residents affected by the fire.

On Thursday afternoon, some of the residents in adjacent blocks who had been evacuated as a precaution were allowed back into their homes.

Volunteers distributed clothes and lunch boxes at the open-air podium of a nearby mall, while a few people gave out flyers with information about missing people.

Deadly fires were once a regular scourge in densely populated Hong Kong, especially in poorer neighbourhoods.

However, safety measures have been ramped up in recent decades and such fires have become much less commonplace.