Plumes of smoke can be seen rising from a large part of the Bwlch mountain.
Eyewitness reports say that the fire has come down as far as the road on the Rhondda Cynon Taf mountain.
Dramatic pictures from the scene show smoke rising from the mountainside, and the mountain road has been closed.
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A local resident said: "The fire starts from a bend on the Bwlch mountain known as Tommy's Bend, and spreads up the mountainside.
"A fire engine can be seen at the starting point of the fire, and cars do appear to be using the road at the moment.
"There is a lot of smoke near the road, but the flames from the fire appear to be further up towards the top of the mountain (and away from the road itself). It's quite windy up here and that appears to be making the smoke spread."
Firefighters can be seen spraying water at the fire from the road.
South Wales Community Rangers said the fire had been reported to them and they were on route to the scene.
Last month, firefighters were faced with 'mile-long' grass fires on the neighbouring Maerdy and Rhigos mountains.
Residents compared the scene as like something from the apocalypse.
