The blaze broke out this morning in the Upperlands area.
A large fire at a derelict building on Kilrea Road, Upperlands has now been dealt with.
The initial call was received at 9.06am this morning (Wednesday, February 11).
The fire, which was dealt with just after 3pm, is being treated as 'deliberate'.
A NIFRS spokesperson said: "Our Firefighters worked tirelessly throughout the duration of the incident to ensure that the fire was brought under control and extinguished.
"At the height of the incident over 50 Firefighters and personnel were involved in the operational response with Appliances from Maghera, Kilrea, Magherafelt, Northland, Coleraine, Dungiven, Strabane, Pomeroy and Ballymena Fire Stations, including 2 water tankers and an Aerial Appliance.
"We would like to thank the public for their patience whilst we dealt with the incident.
"The cause of the fire is believed to have been deliberate ignition and the incident was dealt with by 3.17pm."
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