'Negligent' builders blamed for HK blaze that killed 55

Hong Kong firefighters have brought under control a huge blaze in an apartment complex that killed at least 55 and left nearly 300 missing, while police say 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 2000 apartments home to more than 4600 people in the financial hub struggling to overcome chronic shortages of affordable housing.

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.

"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," said police superintendent Eileen Chung.

Video images from the scene showed flames leaping from at least two of the 32-storey towers sheathed in green construction mesh and bamboo scaffolding.

Authorities said they had doused the flames in four of seven affected blocks, with those in the rest brought under control.

Police said that as well as the protective mesh and plastic coverings of the buildings, which may fall short of fire standards, they found foam material sealing some windows on one unaffected building, and installed by a construction company engaged in year-long maintenance work.

Police have arrested two directors and an engineering consultant of the company on suspicion of manslaughter over the fire, Chung said.

The estate has been under renovation for a year at a cost of $HK330 million ($A65.5 million), media said.

Hong Kong's corruption body said it had launched an investigation into suspected graft related to the renovation.

A firefighter was among the dead, while dozens in hospital were in critical condition, authorities said on Thursday.

About 279 people have yet to be traced.

Two Indonesian migrant workers "in the domestic sector" died in the fire and two others were injured, the Southeast Asian nation's foreign ministry said.

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

More than 1200 firefighters are battling to control the flames, along with 304 fire engines and rescue vehicles.

"The priority is to extinguish the fire and rescue the residents who are trapped," city leader John Lee told reporters, adding that support for the injured and recovery efforts would follow before a thorough investigation was launched.

The fire has prompted comparisons to London's Grenfell Tower inferno that killed 72 people 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."

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 block of the complex during renovation.

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

Wang Fuk Court is one of many high-rise housing complexes in Hong Kong, one of the world's most densely populated cities.

Occupied since 1983, the complex is under the government's subsidised home ownership scheme, according to property agency websites.