Fire at St John's House in Downpatrick
About 30 firefighters are tackling a blaze in Downpatrick.
The fire is at St John's House on Pound Lane, an unoccupied former healthcare building.
Four fire appliances and one aerial appliance are being used to fight the fire, which was reported at about 15:45 BST on Sunday.
Three teenage boys have been arrested on suspicion of arson with intent to endanger life. Nearby residents have been asked to keep windows closed.
Dermot Rooney, from the Fire and Rescue Service, said: "One part of the roof is proving hard to gain access to, so we're using the aerial appliance to flood the fire with water from above.
"The building is not in touch with any other buildings, there's no danger of it spreading to anywhere else.
"They've got the fire surrounded, albeit it isn't properly under control at present. We hope to have it under control shortly."