More than 100 firefighters battled towering flames and thick smoke from a seven-storey blaze in central Sydney on Thursday that was spreading to other buildings.

"The building is starting to collapse, while the inferno is beginning to spread to several neighbouring buildings, including residential apartments," Fire and Rescue NSW said in a statement.

Emergency services said they deployed more than 100 firefighters, with 20 fire trucks, who were "working to contain and extinguish the blaze" on Randle Street near the city's Central Station.

At least one nearby vehicle has been destroyed by fire, they said.

"The public is urged to avoid the area as firefighting operations continue," the fires service said.

Images posted on social media showed flames almost as high as the building itself shooting into the sky.