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Fires are believed to have been deliberately started on a mountain where crews have been tackling a blaze since the weekend.
The fire at Llanwonno, near Ferndale in the Rhondda Valley, began on Saturday afternoon with emergency services remaining at the scene.
It remains a major incident due to the scale of the fire and "challenging conditions" on the mountain, including low cloud, mist and rain.
South Wales Fire and Rescue Service (SWFRS) said there were reports of "deliberate fire setting" on the mountain overnight on Monday, while firefighters continued working to contain the blaze.
Image source, Martin PhillipsImage caption, South Wales Fire and Rescue Service said there were reports of "deliberate fire setting" on the mountain
Locals have not been forced to evacuate, but a rest centre has been set up for people who do not wish to stay at home because of poor air quality.
A spokesperson for SWFRS said it continued to work with partner agencies to contain and extinguish the wildfire on Llanwonno mountain and prevent further spread, helping to protect nearby properties and communities.
"There were reports of deliberate fire setting on the mountain [on Monday night]," they said.
"Firefighters worked throughout the night in challenging conditions to tackle the fire and protect properties."
Resident Caryl Holmes said most of her family live around the area, with her daughter's house "practically on the mountain".
"I've been checking in on everybody making sure the fire wasn't near their homes, but it was near my daughter's," she said.
"It was so close and very frightening to be honest. She wasn't allowed to come home 'til it was put out."
Image caption, Caryl Holmes says the community had been keeping an eye out for each other as the fire continued to affect the village
She said the fire was particularly bad on Monday night when "the smoke was so thick I couldn't breathe" and added that the firefighters had been "amazing".
Ian Putland said even with the windows closed he could smell the fire.
"I heard about it a couple of days ago, and didn't think it would affect me... but then I came out yesterday and saw ash on the car and it indicated how close the fire was," he said.
"I was then alarmed with all the fire people about."
He added that he was "frustrated that the fires had been started deliberately".
"There's the wildlife killed, people are being put in danger and it takes emergency services away. It is a real waste of resources."
Christian Hadfield, area manager at SWFRS, said crews had been at the incident since Saturday but declared it a major incident on Monday in order to protect homes in the area.
"We are putting fire breaks in and monitoring the area," he said.
He added there were reports of "pockets of deliberate fire setting" which put the crews "under really critical conditions and stretches the service".
The fire was "unpredictable and complex work", he said, and firefighters could be there "for many days to come".
Image caption, Christian Hadfield said tackling the mountain blaze was "unpredictable and complex work"
Hadfield said a specialist firefighting helicopter had been secured to help crews fight the blaze but could not be deployed due to the poor weather conditions.
He added that residents affected by smoke were advised to keep windows and doors closed where possible.
SWFRS said road closures remained in place and members of the public were asked to avoid the affected area.
Anyone with any information is being asked to contact South Wales Police.
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