About 35 firefighters tackled the blaze at a derelict hospital building.
At the time the PSNI advised residents to keep doors and windows closed while they deal with the blaze.
The fire was reported at about 7.30pm.
Northern Ireland Fire and Rescue Service group commander Alastair McConville said operations were "hampered" by hazardous materials onsite.
Four fire engines were at the scene along with an aerial appliance, a water tanker and a command support unit.