East Sussex Fire and Rescue Service and Sussex Police were called to the scene.
It happened in Warleigh Road, in Brighton.
A spokesperson for the fire service said firefighters were set to the basement flat fire just after 2.10pm.
The spokesperson added that four fire engines attended and evacuated affected residents.
Nearby residents were asked to keep their doors and windows closed.
East Sussex Fire and Rescue Service added firefighters wore breathing apparatus and used one hose reel jet and one main jet to tackle the fire, which had spread to the first floor of the building.
The fire service issued an update at 3.30pm to say that all persons have been accounted for and both police and ambulance are in attendance.
The spokesperson said: “Electrics have been isolated and UK Power Networks have been requested to attend. Smoke is reported to have affected all three floors of the building.
“The incident has now been scaled back one appliance.”
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Emergency services at the scene of the fire. Picture: Eddie Mitchell Photo: Eddie Mitchell
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Emergency services at the scene of the fire. Picture: Eddie Mitchell Photo: Eddie Mitchell
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Emergency services at the scene of the fire. Picture: Eddie Mitchell Photo: Eddie Mitchell
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Emergency services at the scene of the fire. Picture: Eddie Mitchell Photo: Eddie Mitchell