Walkers have been told to stay away from the Slieve Donard area.
The blaze in the Slieve Donard area started in the early hours of Friday morning.
Dramatic images of the Co Down peak ablaze have sparked concern, with First Minister Arlene Foster tweeting: “This is devastating and tragic. The impact on wildlife and flora is unimaginable. Full support to those battling the flames.”
Northern Ireland Secretary Brandon Lewis described the scenes as “deeply distressing” at one of the region’s most beautiful landmarks. He paid tribute to the fire fighters at the scene.
On Saturday morning the Northern Ireland Fire and Rescue Service (NIFRS) said the are escalating their operations in the Mournes, and described a “large number of resources” in attendance.
The NIFRS has urged the public not to gather in the area, and told walkers to avoid the Newcastle side of Slieve Donard.
A Police Service of Northern Ireland (PSNI) spokesperson said the operation to tackle the fire is expected to be ongoing across Saturday.
“Police would ask that all members of the public avoid the Newcastle area, in particular Trassey Road, Bloody Bridge, Tollymore Forest Park and Donard Carpark to ensure NIFRS have full accessibility to these areas,” they said.
“We understand many people will have made plans to walk the Mournes today but we would ask that you reschedule as you will be turned away upon arrival.”
SDLP MLA Colin McGrath said there had been hope the blaze could be contained on Friday, however light winds had spread the flames.
“By the afternoon of Friday there were some 60 brave firefighters doing their best to maintain the blaze, with the smoke visible from as far away as Downpatrick,” he said.
“I pay tribute to their bravery and determination. Sadly, this will not only have a devastating impact on the natural landscape but the local economy of Newcastle that relies so heavily on the tourist product we associate with the Mournes.”