British fire officials have asked for military help to fight a moorland fire that drove dozens of people from their homes in northwest England on Wednesday.

Greater Manchester Fire and Rescue Service said 100 firefighters were tackling seven pockets of fire across a six-kilometre area of Saddleworth Moor on the outskirts of the city.

One witness said he had never seen anything like it.

"[There were] 20, 30-foot flames coming straight towards us," he said.

Greater Manchester Fire Service spokesman Dave Keelan said he had not seen an incident like this on the moors in Greater Manchester for as long as he could remember.

"It's extremely challenging, [dealing] with a moorland fire like this … with the wind direction, the heat and the terrain, getting firefighters to the places where the fires are, and looking after their welfare," he said.

Assistant chief fire officer Leon Parkes said some of the fire, which has destroyed about 800 hectares of moorland, was in remote, hard-to-reach areas.

Crews are battling the Saddleworth moorland fire amid hot weather and challenging conditions. ( Supplied: Greater Manchester Fire Service )

Dave Keelan, director of emergency response at the fire service, said "we have requested military assistance from the Ministry of Defence".

He said the fire had taken hold in dry peat and was very hard to extinguish.

Firefighters do not yet know the cause of the fire.

Much of Britain is experiencing a heatwave and unusually dry weather.

Smoke from the fire rises from a hill above a town near the moor. ( Supplied: Glossop Mountain Rescue Team )

Brenda Warrington, leader of Tameside Borough Council, said it was "a very changeable situation, almost minute by minute".

"It's dry as a tinderbox up there," with wind fanning the flames, she said.

Residents were told to leave 34 homes in the village of Carrbrook late Tuesday as winds pushed the flames closer.

"The flames were getting closer and closer and the smoke got thicker — you couldn't see anything, you could hear the sparks," said Sue McDowell, who left with her husband Peter, their West Highland terriers and their cat.

"We just grabbed whatever we and could got out. It was scary."

A helicopter drops water as firefighters tackle the blaze. ( Reuters: Phil Noble )

AP