Hong Kong firefighters brought under control on Thursday a huge blaze in an apartment complex that killed at least 55 and left nearly 300 missing, while police said its cause could have been a "grossly negligent" construction firm using unsafe materials.

Rescuers battled intense heat and thick smoke for more than a day after the blaze erupted as they fought to reach residents feared trapped on the upper floors of the Wang Fuk Court housing complex in the northern district of Tai Po.

The eight blocks of the tightly packed complex have 2,000 apartments home to more than 4,600 people in the financial hub struggling to overcome chronic shortages of affordable housing.

A distraught woman carrying her daughter's graduation photograph searched for her child outside a shelter, one of eight that authorities said house 900 residents.

"She and her father are still not out yet," sobbed the 52-year-old, who gave only her surname, Ng. "They didn’t have water to save our building."

On Thursday, police officers searched the building maintenance company responsible for the housing estate, seizing documents that mentioned it, media said. The company did not immediately respond to requests for comment.

Police said in addition to the buildings being covered with protective mesh sheets and plastic that may not meet fire standards, they discovered some windows on one unaffected building were sealed with a foam material, installed by the construction company carrying out maintenance work.

"We have reason to believe that the company’s responsible parties were grossly negligent, which led to this accident and caused the fire to spread uncontrollably, resulting in major casualties," Eileen Chung, a Hong Kong police superintendent, said.

By Thursday morning, authorities said they had brought the fire in four blocks under control, with operations continuing in three blocks.

Video from the scene showed flames still leaping from at least two of the 32-storey towers sheathed in bamboo scaffolding and green construction mesh, as heavy smoke billowed into the sky.

Three men from the construction company, two directors and one engineering consultant, had been arrested on suspicion of manslaughter over the fire, she added.

The green construction mesh and bamboo scaffolding used on the buildings are a mainstay of traditional Chinese architecture but have been subject to a phase-out in Hong Kong since March for safety reasons.

A firefighter was among the 44 killed, with 45 people in hospital in critical condition, Hong Kong police told a press conference before dawn on Thursday.

"The priority is to extinguish the fire and rescue the residents who are trapped," Hong Kong leader John Lee told reporters earlier. "The second is to support the injured. The third is to support and recover. Then, we'll launch a thorough investigation."

Some 279 people were uncontactable and 900 were in eight shelters, he added.

The death toll is now the highest in a Hong Kong fire since 1948, when 176 people were killed in a warehouse blaze.

Searching for relatives

The latest fire has prompted comparisons to the Grenfell Tower inferno that killed 72 people in London in 2017. That fire was blamed on firms fitting the exterior with flammable cladding, as well as failings by the government and the construction industry.

"Our hearts go out to all those affected by the horrific fire in Hong Kong," the Grenfell United survivors' group said on social media. "To the families, friends and communities, we stand with you. You are not alone."

A woman surnamed Ng, 52, was distraught as she looked for her daughter outside a shelter.

"She and her father are still not out yet. They didn’t have water to save our building," she sobbed, carrying her daughter's graduation photo.

Harry Cheung, 66, who has lived at Block Two in one of the complexes for more than 40 years, said he heard a loud noise about 2:45 p.m. (0645 GMT) and saw fire erupt in a nearby block.

"I immediately went back to pack up my things," he said.

"I don't even know how I feel right now. I'm just thinking about where I'm going to sleep tonight."

Another long-time resident, a woman surnamed Chu, said she still had not been able to contact her friends who live in the next block. After staying over at a friend's place on Wednesday night, the 70-year-old came back to see her home still burning.

"We don't know what to do," she said.

An online app showed missing persons reports submitted through a linked Google document that detailed residents of individual towers and rooms.

It includes descriptions like "Mother-in-law in her 70s, missing" or "one boy and one girl" or "Rooftop: 33-year-old male."

One description simply says "27th floor, room 1: He is dead." Reuters could not independently verify the information on the app.

China's Xi urges 'all-out' effort against fire

Many residents took to social media to criticise what they saw as negligence and cost cutting as a cause of the fire. One video showed several construction workers smoking on the bamboo scaffolding surrounding one of the complex's blocks during the renovation process.

From the mainland, China's President Xi Jinping urged an "all-out effort" to extinguish the fire and to minimise casualties and losses, China's state broadcaster CCTV said.

Hong Kong's Transport Department said that a number of roads would remain closed in the area on Thursday morning and 39 bus routes have been diverted.

At least six schools will be closed on Thursday due to the fire and traffic congestion, the city's Education Bureau said.

On Wednesday, frames of scaffolding were seen tumbling to the ground as firefighters battled the blaze, while scores of fire engines and ambulances lined the road below the development.

Hong Kong's government moved to start phasing out bamboo scaffolding in March, citing worker safety after 22 deaths involving bamboo scaffolders between 2019 and 2024. It announced that 50% of public construction works would be required to use metal frames instead.

Though fire hazard was not cited as a reason for the phase-out, there have been at least three fires involving bamboo scaffolding this year, according to the Association for the Rights of Industrial Accident Victims in Hong Kong.

Hong Kong's sky-high property prices have long been a trigger for social discontent in the city and the fire tragedy could further stoke resentment towards authorities ahead of a city-wide legislative election in early December.

Wang Fuk Court is one of many high-rise housing complexes in Hong Kong, one of the most densely populated areas in the world. Tai Po, located near the border with mainland China, is an established suburban district with some 300,000 residents.

Occupied since 1983, the complex is under the government's subsidised home ownership scheme, according to property agency websites. According to online posts, it has been undergoing renovations for a year at a cost of HK$330 million ($42.43 million), with each unit paying between HK$160,000 and HK$180,000.