Firefighters from Tyne and Wear have been helping colleagues battling the huge blaze in Lancashire.
Crews have returned for a seventh day as a "rapidly developing, aggressive fire" ravages moorland across countryside in the county.
Two large-scale moorland fires merged on Saturday due to increasing wind speed and a major incident was declared as crews tackled the blaze.
Greater Manchester Police said fires on Winter Hill and Scout Road near Bolton have merged, with pedestrians and motorists urged to stay away from the scene.
Lancashire Fire and Rescue Service said 10 crews remained on the scene overnight, but the number was increased to 22 from 4.30am on Sunday.
Tyne and Wear Fire and Rescue Service has sent two crews to assist with the blaze.
The service posted on Facebook: "Two of our crews are on Winter Hill, to support colleagues at Greater Manchester Fire and Rescue Service and Lancashire Fire and Rescue Service (Official).
"Stay safe everyone!"
Greater Manchester Fire and Rescue Service (GMFRS) said on Saturday it had 28 fire engines tackling the moorland fires.
There were approximately 120 personnel on the moors, split between seven areas of wildfire in Tameside and Winter Hill, Bolton.
GMFRS returned to tackling the moorland fires in Tameside while colleagues face another tough day doing the same at Winter Hill.
The service tweeted: "The object of today is to continue to control and if possible reduce both incidents. Some people have reported smelling smoke or ash settling on their properties."
The first moorland fire - on Saddleworth Moor - started on Sunday amid Britain's heatwave and has devastated surrounding land around Carrbrook.
Assistant chief fire officer Dave Keelan said: "This is an exceptionally challenging time and I am proud of the hard work and brave commitment of our firefighters."
Lancashire fire service area manager Tony Cook said "very intensive firefighting" is taking place on Winter Hill, including personnel doing shuttle runs in appliances to get water to the scene while others manually try to beat the fire out.
Fire break trenches are also being dug to try to protect local buildings.