Firefighters on Wednesday morning continued their attempts to put out a massive blaze at a Tin Shui Wai building site, more than 12 hours after workers were forced to flee the area.

The Fire Services Department deployed robots and drones, but officers were unable to access the basement where the fire is believed to have started, and which saw a number of explosions.

The fire was upgraded from a three-alarm to a four-alarm blaze on Tuesday evening, the second highest level in the five-grade system.

Officers said 262 firefighters and 11 ambulance staff had been deployed to the scene, adding that people living nearby should keep their windows closed.

There was concern about two 50-metre cranes on the site, though firefighter said they were not in imminent danger of collapse.

A preliminary investigation has revealed that the fire was started by a spark from metal cutting equipment. About 200 workers evacuated the scene, with no reports of injuries.