Firefighters in Hong Kong were still putting out a massive fire on Thursday after a high-rise housing complex went up in flames, killing at least 44 people and leaving hundreds unaccounted for, according to officials.

The fire, described as the city’s worst in decades, began on Wednesday afternoon in an eight-building estate with about 2,000 flats.

Police said early Thursday that three men had been arrested. Investigators believe flammable materials left behind during maintenance work caused the fire “to spread rapidly beyond control”.

Officers also searched another location and carried away folders linked to the arrested men, whom police suspect “acted with gross negligence” by leaving foam packaging at the site.

The fire started on bamboo scaffolding across several 31-storey blocks in Wang Fuk Court in Tai Po, where large-scale repairs were underway.

Reporters at the scene saw thick smoke and ash rising high into the sky as parts of the bamboo crackled loudly.

A long-time resident, a 65-year-old man surnamed Yuen, said many elderly people live in the complex and may not have been able to move quickly.

He said some neighbours even needed phone calls from others to realise there was a fire because their windows were shut for maintenance.

“The windows were closed because of the maintenance, (some people) didn't know there was a fire… I'm devastated,” Yuen told AFP.

The city’s fire department confirmed the death toll had risen to 44.

A 37-year-old firefighter died after losing contact with his team. He was later found with burns on his face.

Hospitals are treating 56 injured people, including 16 in critical condition and 24 seriously hurt.

Hong Kong leader John Lee said early Thursday that 279 people were unaccounted for, though firefighters later managed to reach some of them.

More than 900 residents took shelter in temporary centres set up by the government.

A police officer at one shelter said many families were still arriving to report missing relatives, making it hard to know the exact number of people still not located.

Deputy fire operations chief Derek Armstrong Chan said some areas were too hot to enter.

“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,” he said.

He added that strong winds and burning debris likely pushed the flames between buildings, but officials are continuing their investigation.

Chinese President Xi Jinping sent condolences to the families of victims, including “the firefighter who died in the line of duty”, state media reported.

Hong Kong’s leader, John Lee, said he was “deeply saddened” and that government teams were helping affected residents.

A Tai Po resident surnamed So, 57, said the scenes were heartbreaking.

“There’s nothing that can be done about the property. We can only hope that everyone, no matter old or young, can return safely,” he told AFP.

Another resident, an apartment owner in his 40s, said he felt unsafe leaving his building while the fire was still spreading.

“The fire is not yet under control, and I dare not leave, and I don't know what I can do,” he said.

Large buses were used to evacuate residents, and nearby buildings were also cleared as a precaution.

Parts of a nearby highway had to be shut down for the firefighting operation.

Hong Kong once saw frequent deadly fires due to its crowded buildings, but safety standards have improved in recent years.

This week’s deadly blaze has shocked the city and raised concerns about fire safety during building repair work.