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 spreading to 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 four buildings as flames and ash reached high into the sky.

The blaze showed no signs of slowing after dark, with the buildings engulfed in an eerie orange glow.

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

However, 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 Information Services Department said four people were certified dead and three were in hospitalised in critical condition as of 6:00 pm (1000 GMT).

Two other people were also hurt, it said, one seriously.

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

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 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.