Six fire engines and 32 firefighters have tackled a blaze at “sheltered dwellings” in Portstewart, the fire brigade have said.
One casualty was reported – a male who had suffered smoke inhalation.
They were taken to hospital.
The BBC has reported that their injuries are not life-threatening.
The fire began at the building in Mill Road, Portstewart, at about 7.45pm.
Though the fire brigade said it sheltered accommodation, it did not specify who was housed there.
The fire brigade said in a statement: “On arrival, the initial fire crew discovered a well-developed fire within a flat which went on to involve one wing of the building.
“A total of five fire appliances, one aerial appliance and a total of 32 firefighters attended and quickly brought the incident under control. “Unfortunately one wing of the building has been severely damaged.
“The fire was prevented from spreading to the remainder of the building which remains in use.
“Residents were evacuated by staff and firefighters to safe areas of the building and the local Presbyterian church has provided shelter and support for those affected.”