Seven people were injured, six of them seriously, in a fire that broke out in a Mong Kok residential building early on Friday morning.

Firefighters said one man was unconscious when they found him in a smoke-filled corridor on the 10th floor of Cheung Fai Mansion on Tong Mi Road. He was described as being in a critical condition.

The man and six other people were given oxygen as they were taken to hospital.

Senior station officer Sz-to Wang-pan said the building's fire alarm did not go off.

"The building is undergoing a fire safety renovation project, so [that is] possibly why the fire alarm didn't go off. And the heat of the fire also affected electric cables of the alarm system in the corridor, so that may also be a reason that the alarm system wasn't activated," he told reporters.

He said the fire, which was reported shortly after 6am, was put out in under an hour. It was not thought to have been started deliberately.

"The fire was the most intense in the living room, and we aren't ruling out that some electrical appliances there first caught fire and the flames spread. But we need to investigate further," Sz-to said.