Dave Keelan, director of emergency response at Greater Manchester Fire and Rescue Service, said: "We haven't seen an incident like this on the Moors on Greater Manchester as long as I can can remember."
He added: "We have been liaising with a military adviser on the scene and following those discussions we have requested military assistance from the Ministry of Defence.
"There's still a lot of smoke from the fire, but air quality levels are being monitored regularly in different locations. Air quality is currently at a safe level and therefore residents have been let back into their homes.
"We don't know the cause of this incident at this stage and it would be very difficult to ascertain the cause due to the extent of the damage but we will be looking into that once we have tackled the fire."
Speaking about the fire-fighting efforts, Keelan said: "They are working extremely hard, as they have been for the past couple of days, in really arduous conditions in heat and smoke, trying to extinguish the fire.
"At the minute we have four different areas of fire that is in the moorland and embedded within the peat which is extremely difficult to extinguish.
"We have got really good water supplies set in place now and we have got a really good tactical and strategical plan in place.
"The crews are working in shifts and rotating in these conditions to extinguish the fire as quickly and as safely as they can do."
Brenda Warrington, leader of Tameside Borough Council, admitted they now needed "Mother Nature's help", saying that only a "really really good downpour" would extinguish the blaze.
However, BBC weather forecaster and meteorologist Simon King, based at Salford Quays, where smoke infiltrated the studios, said: "Nature isn't going to help put out the fires on Saddleworth Moor.
"Another hot day today with a brisk easterly wind, especially this evening. Little to no rain in the forecast for at least a week either."
The fire brigade said they were monitoring air quality and would evacuate more residents if pollution and particulates rose to dangerous levels, while PHE warned asthmatics and people with a heart condition to stay indoors and everyone was told to keep windows and doors shut.
Theresa May said the Home Office was monitoring the situation closely with the National Resilience Assurance Team and environmental experts said that the impact on health could be severe.
Dr Thomas E L Smith, Assistant Professor in environmental geography at the London School of Economics and Political Science, said: "Photos from the eastern suburbs of Manchester suggest 'hazardous' levels of particulate air pollution, and we have data from Manchester Piccadilly that indicates 'unhealthy' levels in the city centre."
Prof Alastair Lewis, Professor of atmospheric chemistry at the National Centre for Atmospheric Science, University of York, said: "The particles from this kind of burning are characteristic in that they can carry on their surfaces a range of toxic chemicals.
"Reducing public exposure to these cancer-causing chemicals was one of the primary motivations behind the banning of stubble burning in the 1990s."
