Hong Kong's deadliest fire in years has killed at least 36 people, including a firefighter, with another 279 people reported missing, Hong Kong leader John Lee says.

He said the fire, which broke out at a residential complex in Tai Po, a suburb in the New Territories, was"coming under control" shortly past midnight (3am AEDT) and 29 people remained hospitalised.

Hundreds of residents were evacuated as the blaze spread across seven high-rise apartment buildings on Wednesday.

Fire chiefs said high temperatures at the scene made it difficult for crews to mount rescue operations. It was not immediately known how the fire started.

The raging fire sent up a column of flames and thick smoke as it spread quickly on bamboo scaffolding and construction netting that had been set up around the exterior of the housing complex.

Records show the housing complex consisted of eight blocks with almost 2000 apartments housing about 4800 people.

Multiple buildings close to each other were set ablaze, with bright flames and smoke shooting out of windows as night fell. Authorities said that hundreds of firefighters, police officers and paramedics were deployed. Firefighters aimed water at the intense flames from high up on ladder trucks.

The blaze, which started mid-afternoon, was upgraded a level 5 alarm — the highest level of severity — as night fell. Authorities said that conditions remained very challenging for firefighters.

"Debris and scaffolding of the affected buildings (is) falling down," Derek Armstrong Chan, deputy director of Fire Service operations, said.

"The temperature inside the buildings concerned is very high. It's difficult for us to enter the building and go upstairs to conduct firefighting and rescue operations."

Officials said the fire started at the external scaffolding of one of the buildings, a 32-storey tower, and later spread to inside the building and also to nearby buildings, likely aided by windy conditions.

The fire department said that it received"numerous" calls requesting assistance. It said some residents remained trapped as of Wednesday night, but police declined to provide details about how many were missing or in danger.

Firefighters deployed 128 fire trucks and 57 ambulances to the scene.

The dead included a 37-year-old firefighter, while another received treatment for heat exhaustion, Director of Fire Services Andy Yeung said.

Chinese leader Xi Jinping on Wednesday expressed condolences to the firefighter who died and extended sympathies to the families of the victims, according to state broadcaster CCTV. He also urged efforts to minimise casualties and losses.

District officials in Tai Po have opened temporary shelters for people left homeless by the fire.

"I've given up thinking about my property," a resident who only provided her surname, Wu, told local TV station TVB.

"Watching it burn like that was really frustrating."

Tai Po is a suburban area in the New Territories, in the northern part of Hong Kong and near the border with the mainland Chinese city of Shenzhen.

Bamboo scaffolding is a common sight in Hong Kong at building construction and renovation projects, though the government said earlier this year that it would start phasing it out for public projects because of safety concerns.

The fire is the most deadly in Hong Kong in years. In November 1996, 41 people died in a commercial building in Kowloon in a level 5 fire that lasted for around 20 hours.