The inferno -- the financial hub's worst in decades -- sent shock waves through the city, which has some of the world's most densely populated and tallest residential blocks.

Here's what we know about the fire and its possible cause:

- Inferno -

Intense flames burned on bamboo scaffolding on several apartment blocks of Wang Fuk Court, a residential complex undergoing repairs in the northern district of Tai Po, on Wednesday.

The blaze rapidly engulfed several towers at the housing estate, which includes eight buildings of 31 floors each and that have a combined total of 1,984 units.

Firefighters, one of whom was killed, battled the inferno overnight and it was still burning in spots on Thursday morning.

Hong Kong's fire department said at least 44 people have died and authorities have said hundreds remain missing.

Around 900 residents have been moved to temporary shelters and dozens were in hospital, some in critical condition.

-Tightly packed city-

Hong Kong is one of the most densely populated places in the world, making urban disasters a significant risk.

Its 7.5 million residents are squeezed into islands covered in steep hills.

The city's population density is more than 7,100 people per square kilometre of land, on par with packed metropolises like Tokyo.

-Vertical living-

The financial hub is famous for its dramatic skyline of sky scrapers set against a picturesque harbour, many of which are home to residents as well as as banks and commercial offices.

The city's construction boom in past decades has been largely fuelled by residential towers to house its growing population.

Much of the new residential development in recent decades has been in the New Territories, the area where Tai Po is located.

Hong Kong has 569 buildings above 150 metres, the most in the world, according to a tracker from the Council on Tall Buildings and Urban Habitat.

- Bamboo, wind, sheeting -

The original source of the fire is not clear but investigators were already probing the installation of flammable materials during the construction on the buildings as a potential factor in the dramatic escalation of the blaze.

Police have arrested three men from the construction company involved, accusing the firm of gross negligence leading to the accident and causing the fire "to spread rapidly beyond control".

The external walls of the soaring residential towers were covered in bamboo scaffolding and wrapped in netting and plastic sheeting.

Police said after preliminary investigations they suspected many of those materials did not meet fire safety standards.

Investigators also found packaging foam at the site which they said was highly flammable and could have contributed to the quick progression of the blaze.

Breezes of around 14 kilometres (nine miles) an hour were recorded in the area around the time the fire started on Wednesday afternoon.