A spokesman from East Sussex Fire and Rescue Service said four engines were sent to a block of flats in Wellington Road, Brighton at 1.32pm today.
After clearing the building, one engine left, leaving three to work on the fire, the spokesman added, which was believed to have started in the roof.
All crews left the scene at 3.20pm, said the spokesman.
The exact cause of the fire is not known at this stage and an investigation is ongoing.
Footage from the scene shows a firefighter standing on top of the building, spraying water directly onto the fire.