United Kingdom — Firefighters were called to the Penderyn area at 0850 BST on Thursday.
The 500-hectare blaze, which includes the Storey Arms area, had a front of 10 miles (16 km) at one stage, but is now said to be largely under control.
Four fire engines, a helicopter, water bowser and all-terrain vehicles were dispatched.
BBC reporter Nick Servini said from the scene: “We’ve been told, from the police and the fire service, that they believe this fire was started deliberately.
“There have been a number of incidents recently and this was one of them and, like so much of the country, up here it’s bone dry at the moment.
“I can’t remember when it last rained and it’s surprisingly windy, so it’s ideal conditions for the fire to spread.”
A triangular area between Llwyn-Onn and Cantref reservoirs and the village of Penderyn has been affected.
The A4059 road between Cantref and Penderyn reopened after being closed for several hours due due to smoke.
A number of farmers helped to move livestock out of the affected area. Earlier they had to fight the fire close to the Garw Nant visitor centre.
Fire crews were sent from Hirwaun, Aberdare, Merthyr Tydfil and Blaenavon.
Water from a local reservoir was carried by helicopter and used to tackle the blaze.
High winds have caused the fire to “spread quickly” and because the land is boggy it “gives out a lot of thick, black smoke”, according to Nigel Williams, a station commander for South Wales Fire and Rescue Service.
The fire service said a “large number” of grass fires have taken place in the south Wales valleys over the last few days, many of which are “believed to have been started deliberately”.
Martin Henderson, the service’s head of risk reduction, said both the police and fire service “will pull out all the stops” to ensure anyone who starts fires are caught.
He said: “The people responsible for starting these fires need to understand that the fires severely damage much of the countryside surrounding their communities and they are putting lives in danger as it can take us a few minutes longer to respond to incidents in the towns and villages of south Wales if we are on top of a mountain tackling a forest fire.”