Around 1200 people have been evacuated in Sydney's CBD due to a fire in a multi-story building.

The area around World Square is engulfed with smoke from the blaze, which firefighters believe may have started in an oven in a pizza shop on the ground floor.

About 50 firefighters are on the scene at 338 Pitt St.

Workers and residents on the 14th floor and above remain inside the building following directions from Fire and Rescue NSW.

Superintendent Ian Krimmer says they are not in danger and can be evacuated if necessary.

"There is no risk whatsoever," Supt Krimmer said.

There have been no reports of any injuries.

The areas around Pitt St and Liverpool St have been closed.

Witnesses said they could smell "burning brake oil" in the minutes before the blaze and they called 000 when they saw smoke billowing from a vent.

"Within a couple of minutes it was a big inferno," Tracie Mitchell told AAP.

"Smoke was everywhere. It looked like when the buildings collapsed at 9/11."