The fire, which started in the afternoon and was still burning late into the night, was the deadliest in decades in Hong Kong, which has some of the world's most densely populated and tallest apartment blocks.

Massive flames first took hold on bamboo scaffolding on several apartment blocks of Wang Fuk Court, which contains nearly 2,000 flats in eight towers and was reportedly undergoing estate-wide maintenance.

An AFP reporter heard loud cracking sounds, possibly from the burning bamboo, and saw thick plumes of smoke billowing from the buildings as flames and ash reached high into the sky.

A 65-year-old resident surnamed Yuen said he had lived in the estate for more than four decades and that many of his neighbours were elderly and might not be mobile.

"The windows were closed because of the maintenance, (some people) didn't know there was a fire and had to be told to evacuate via phone calls by neighbours," Yuen told AFP.

"I'm devastated, there is loss of property and loss of life, and even a firefighter has died."

Nine people died at the scene and four more, including a 37-year-old firefighter, were certified dead at hospital, the government said late on Wednesday.

A police officer at a temporary shelter told AFP it was unclear how many people were unaccounted for because residents were still trickling in late into the night to report missing family members.

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

Hong Kong's hospital authority said 23 people were injured, with six in critical condition and 11 listed as serious.

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

"He offered sympathies to the families of the victims and those affected by the disaster, and called for making every effort to extinguish the fire and minimise casualties and losses," state broadcaster CCTV said.

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

- 'Dare not leave' -

A Tai Po resident surnamed So, 57, earlier told AFP near the scene that the fire was "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," So said.

An apartment owner in his 40s who did not want to give his name told AFP that the government needed to help those made homeless by the blaze.

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

Residents were seen being evacuated via large coaches, with local media reporting that adjacent blocks were also being cleared.

Authorities set up a casualty hotline and opened two temporary shelters in nearby community centres.

Sections of a nearby highway were also closed by the firefighting operation.

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.

The Association for the Rights of Industrial Accident Victims expressed "deep concern" over scaffolding-related fires, noting similar incidents in April, May and October.

Four people were hospitalised after a separate fire on the scaffolding of a building in Hong Kong's central business district last month.