Arsonists have been spotted lighting new fires near Bolton even though a huge moor fire is continuing to burn, Sky News has been told.
It is "absolutely heartbreaking and unbelievable", Tony Crook from Lancashire Fire and Rescue Service said.
The perpetrators are putting "yourselves at risk, your community at risk, and also putting emergency workers at risk", Mr Crook added.
In a further message for them, he said: "You will get caught, and you will get dealt with."
A week-long battle against the flames was declared a major incident on Saturday.
On Sunday, when a police helicopter was sent up to survey the area, people were spotted lighting fires in grassland called Healey Nab.
"As they've been trying to extinguish the fire, people have been seen lighting other grass fires in the vicinity," Mr Crook said.
"Police are now attending that scene, to try to apprehend those arsonists. It absolutely astounds me."
Fire crews were already tackling "rapidly developing, aggressive" wildfires after two huge blazes on Winter Hill and Scout Road merged into one.
They have launched a "large scale attack" against the flames, and Mr Crook said about 30 fire appliances had been called in.
Pedestrians and motorists are urged to stay away.
Winter Hill is a key communications hub for the North West, and the fire service has been "working with partners to secure the road and infrastructure up on the top of the hill at the mast sites", a spokesman said.
Fire break trenches are being dug to try to protect local buildings.
Other fire brigades from across the country – including Tyne and Wear, the South West, Nottinghamshire, Humberside and Warwickshire – are on the scene to help out.
Met Office weather forecaster Helen Roberts said the hot spell was set to continue.
"The trend is for dry and very warm weather, which is far from ideal," she said.
"Thunderstorms in the South West are unlikely to reach that far north.
"On Wednesday, there is the potential for showers in that vicinity but it could be a one-off, isolated and unlikely to hit the area.
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"Thursday is similar and any showers would be isolated and fleeting.
"There is no significant rain forecast this week."