Irish Air Corp helicopters are deployed to assist firefighters as they continue to tackle the blaze.
The NI Fire and Rescue Service (NIFRS) has declared the continuing fires on the Mourne Mountains in County Down as a “major incident”.
An Irish Coast Guard helicopter was deployed to assist with the operation on Saturday afternoon.
Police have urged the public to stay away from the area to allow crews to concentrate on the blaze.
NI Secretary of State Brandon Lewis asked people to heed this advice as “deeply distressing scenes” unfold in the area of outstanding natural beauty.
The emergency fire operation was escalated on Saturday morning.
The fire is on the lower slopes of Slieve Donard, the Glen River Valley and Thomas Mountain.
Slieve Donard is the highest mountain in Northern Ireland.
NIFRS Asst Ch Fire Officer Aidan Jennings said up to 70 firefighters were involved in tackling the gorse fire, which has spread down the mountains towards forested areas.
He said significant resources would move into the area throughout Saturday with water and foam used to prevent the spread of flames.
Mr Jennings said operations had been set up in Donard Park with crews expected to continue to battle the fire throughout Saturday.
Conventional fire appliances have been unable to reach the fire due to the challenging terrain.
Mr Jennings said firefighters were “taking up shovels and beating out the flames themselves” on the mountains.
He said there has been no decision yet on the cause of the fire.
He strengthened a previous appeal for walkers and campers to stay away from the area.
Mr Jennings said they were battling a “significant gorse fire” in challenging terrain.
“We have a major fire burning, it is extremely visible. We have extreme heat and the crews are having to walk in with their gear, it is difficult to walk in in normal conditions.
“My priority is to protect life and property, no property is immediately at risk. My concern is my firefighters.”
In a statement, the PSNI asked the public to 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”.
It added: “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.”
Mr Lewis paid tribute to the “heroic work” of the emergency services and said the impact of the fire on one of Northern Ireland’s most beautiful landmarks was “tragic”.
At the scene – BBC News NI’s Catherine Morrison
It’s an unusual sight- the top of Slieve Donard obscured not by cloud but by plumes of smoke.
Gorse fires are continuing to spread across the mountain, above the tree line, fanned by a steady eastern breeze.
Dozens of firefighters and crews from as far away as Belfast and Glengormley are tackling the blaze, building firebreaks to stop the fire spreading into the forest.
Many of the most popular walking areas are affected: the Glen river walk, the Trassey track, Bloody Bridge.
On a sunny Saturday like this, there would usually be hikers and mountain bikers taking in the beautiful views.
But today, the warning from police and fire crews for people to avoid the area appears to have been heeded. Donard car park is virtually empty and the town is much quieter than normal.
Police have set up road blocks on the outskirts of Newcastle and are stopping drivers to warn them to stay away from Donard.
In a tweet, First Minister Arlene Foster said the fire was “devastating and tragic” and that she offered her “full support for those battling the flames”.
Deputy First Minister Michelle O’Neill echoed calls for people to avoid the Slieve Donard area and said her thoughts were with firefighters battling “one of our worst ever wildfires”.
The NI Fire and Rescue Service has said that there was potential for further fires this weekend and urged people to avoid the Bloody Bridge and Glen Road access points.
Health Minister Robin Swann said he had spoken to the NIFRS to pay tribute to the crews fighting the blaze.
He joined the appeal to the public to stay away from the area to “allow the crews to concentrate in the job in hand and stay safe”.
The NIFRS has said the area is unsuitable for wild camping, adding “we would ask the public to exercise due care and vigilance in relation to fire safety if out in the countryside during this current dry spell”.
The sight of the Mournes in flames has upset many who love these mountains.
They are an important protected habitat for plants and animals.
Ground nesting birds and insects will have been badly affected.
How quickly they recover will depend on how deeply the fire has penetrated the peat.
Upland areas like this have huge potential to store carbon.
They’ve been described as our equivalent of the rainforest.
And today, they’re once more on fire.
On Friday, Sinn Féin MP Chris Hazzard thanked the NIFRS but said local agencies need “urgent help”.
“Whether accidental or deliberate, wildfires are incredibly dangerous. Today’s fire will have caused widespread damage to an environmentally sensitive area, and will also unfortunately have proven deadly for local wildlife,” he said.
SDLP MLA Colin McGrath paid tribute to the firefighters’ “bravery and determination”, adding that the fire was visible from as far away as Downpatrick.
“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,” he said.
Speaking during a visit to the scene, DUP Agriculture Minister Edwin Poots thanked first responders and said he was “shocked” to see the damage inflicted on the landscape.
“Wildfires cause a devastating level of damage to our natural environment and threaten people’s lives, homes and businesses,” he said.
He added that wildfires were “rarely natural” and were almost always started “either deliberately or by reckless burning of flammable vegetation or material”.