Several residents of a Mong Kok building in which a fire killed three people on Thursday night said the fire alarm failed to go off.

Firefighters went to Kin Wong Mansion on Sai Yee Street around 11pm after receiving reports of the blaze that claimed the lives of a six-year-old boy and two men aged 53 and 80.

A woman who lives in the unit opposite the one in which the fire started said she heard no alarm, despite fire drills being routinely carried out in the building.

She was alerted to the fire when white smoke entered her flat through the entrance.

Trapped with her daughter and their dog by heavy smoke, she was advised by firefighters to stay inside the flat.

"The firefighters had already arrived and they knew I was inside my home," she said.

"They instructed me to go back inside because they had work to do – like breaking down doors. I believe that since they knew I was at home, then I was safe.

"From what I saw, there wasn't too much smoke coming into my home so I just kept out of the way," she said, adding they would stay with a relative until the residual smell dissipates.

Another resident, Kwok, also said the alarm did not go off and that he only realised the gravity of the situation when smoke filled his flat.

He credited the authorities' swift response with preventing an even worse tragedy, noting the situation could have escalated rapidly.

"Our air-conditioner was emitting a lot of smoke. My daughter asked, 'Dad, why is this happening?'" Kwok said.

"I said, 'Did the air-conditioner catch fire?' I immediately turned off the air-conditioner and unplugged it. Then I went to the kitchen to see why it was so smoky. I opened the window to look and saw a lot of smoke," he said.

"I quickly went downstairs. When I got downstairs, I realised that many people from the building had already come out. Lots of them were saying, 'Why is this happening?' We actually know the elderly people from that unit. It became a situation where, once the fire had started, no one would let us go back up."

Chan, a resident living above the unit that caught fire, said billowing smoke forced his family to evacuate.

"There was a lot of smoke and it all came up. The firefighters came to break down doors to see which unit it was and to see if there was anyone who was unconscious," he said.

"With so much smoke here, we definitely needed to evacuate. The firefighters told us to go downstairs and wait," he said.

Firefighters and government laboratory personnel returned to the building early on Friday to continue their investigation.