The Associated Press news agency reports that thick smoke is still pouring out of the Wang Fuk Court complex in Tai Po district in Hong Kong.
Firefighters aimed water at the intense flames from high on ladder trucks, but conditions for fighting the fire and rescuing people remained challenging, the AP said.
Firefighters said the operation to put out the blaze could last until at least Thursday evening.
The housing complex consists of eight buildings with almost 2,000 apartments for about 4,800 residents, including many older people. It was built in the 1980s and had been undergoing a major renovation, according to the AP.