A fire at a residential building in Mong Kok on Thursday night has claimed the lives of three people believed to be from the same family, including a six-year-old boy.

A 75-year-old woman thought to be a relative of the deceased was injured in the blaze.

Firefighters rushed to Kin Wong Mansion on Sai Yee Street after receiving a report of a fire there at around 11pm.

There, they found an injured 75-year-old woman asking for help. She was sent to hospital.

Firefighters then made their way into a burning flat, where they found a six-year-old boy and a 53-year-old man in the bathroom, along with an 80-year-old man in the kitchen. All of them were unconscious, and were later certified dead after they were taken to Kwong Wah Hospital.

Two cats were also found dead inside the unit.

Police later told reporters they believe the three victims of the blaze, along with the rescued woman, are all from the same family.

Forty-eight people were evacuated from the building.

At a briefing, the Fire Services Department said 86 firefighters and medical personnel were deployed to the scene. But they said they weren't able to enter the flat for investigations immediately because of the high temperatures, and will conduct a probe when flat cools down to try to figure out what had started the fire.