A weekend blaze at the old Hilden Mill in Lisburn has destroyed a section of the roof.
The fire broke out in the derelict clothing factory in the Bridge Street area on Friday evening.
Police attended the scene and asked motorists and pedestrians to avoid the area.
Nearby residents were also advised to keep their doors and windows closed.
The Northern Ireland Fire and Rescue Service (NIFRS) said the fire was started deliberately.
Firefighters worked overnight to tackle the blaze, which was brought under control at about 9am on Saturday.
NIFRS Group Commander David Harbinson said the fire started on the fifth floor and spread to the roof.
He described the incident as "challenging" due to the dangerous nature of the derelict site.
Mr Harbinson said 40 firefighters tackled the flames, along with six pumping appliances and an aerial appliance.
Meanwhile, police said they have been dealing with multiple reports of young people entering the site since fire crews left the scene.
They urged parents to explain the dangers of entering the building to their children.
The Hilden Mill has been a local landmark since the 1800s but the site has been vacant for a number of years.
Alliance Cllr Sorcha Eastwood described the incident as “heartbreaking”.
Just devastating to see our heritage & history up in flames 😢 i know my colleague @smartin123 has been working on redevelopment & reimagining this site, but it is heartbreaking to see it up in flames like this https://t.co/vviSik93xd— Sorcha Eastwood MLA (@SorchaEastwood) May 21, 2021
SDLP MLA Pat Catney said the fire is a devastating loss of heritage.
Mr Catney added: “The old Hilden Mill is a landmark piece of history and heritage in our community. Efforts have been ongoing for some time to bring it back into active use and reimagine the site. The blaze over the weekend is a devastating setback for these plans and it’s the local community that will lose out.
“I want to thank fire fighters who have been battling the blaze overnight and would encourage local people to follow the directions of police until the site is made safe.”
The historic mill incorporates more than 24 acres and a number of listed historic buildings, as well as woodland and grassland.
There have been a number of proposals for the site, including a plan by Clanmil Housing Association to develop hundreds of social homes.
One of the buildings, the Barbour Threads Mill, was damaged by fire in 2015.
Exactly ten years ago the mill was saved from destruction after another fire broke out.