Massive flames first took hold on bamboo scaffolding on several apartment blocks of the Wang Fuk Court estate in Tai Po, a district in the northern part of the Chinese financial hub, before engulfing other parts of the buildings.

An AFP reporter heard loud cracking sounds, possibly from the burning bamboo, and saw thick plumes of smoke billowing from at least five out of the estate's eight buildings as flames and ash reached high into the sky.

"There's nothing that can be done about the property. We can only hope that everyone, no matter old or young, can return safely," a Tai Po resident surnamed So, 57, told AFP near the scene of the fire.

"It's heartbreaking. We're worried there are people trapped inside."

The blaze showed no signs of slowing after dark, with flames inside the tower blocks casting an eerie orange glow on the surrounding buildings.

Police said earlier they had received reports of residents trapped in buildings, according to Hong Kong media reports.

Officers at the scene told AFP on the condition of anonymity that they were unable to confirm whether there were still residents stuck in the buildings by nightfall, adding that "firefighters aren't able to go in".

The government's Fire Services Department said 13 people had died, including nine at the scene and four who had been taken to hospital.

Another 15 people were injured, with two hospitalised in serious condition, it said around 8:15 pm (1215 GMT).

A 37-year-old firefighter lost contact for around half an hour and was found with burns on his face, and was certified dead after being rushed to hospital, according to Director of Fire Services Andy Yeung.

Authorities declared a five-alarm fire -- the highest level -- as night fell.

- 'Dare not leave' -

A residential unit owner in his 40s 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.

The South China Morning Post newspaper said police had begun evacuating two buildings in another residential estate nearby.

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

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

"Residents nearby are advised to stay indoors, close their doors and windows, and stay calm," the Fire Services Department said.

"Members of the public are also advised to avoid going to the area affected by the fire."

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

Hong Kong has some of the world's most densely populated -- and tallest -- apartment blocks.

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.

Authorities have not yet spoken about the possible causes of the blaze.