Police said that evacuations had been carried out as a precaution, and that the C7 road – known locally as Ski Road – has been closed.
A spokesperson for the force said: “Anyone walking in the Cairngorms should avoid the area and let family or friends know when they have returned.”
Earlier the Royal Society for the Protection of Birds (RSPB) sent all staff on duty to the scene, as the blaze is on the southern boundary of the Loch Garten Nature Reserve in Abernethy, which it owns.
RSPB Scotland said approximately 300 hectares of land had been burned so far.
The charity said 27 of its staff and volunteers were tackling the blaze on Wednesday, using equipment including all-terrain vehicles with water bowsers, backpack hoses and shovels.
The fire is not a risk to ospreys that are located about six miles away, an RSPB Scotland spokesperson said.
However, BBC Scotland News understands there are capercaillie – a large woodland grouse – around Ryvoan.
A number of campers and residents in nearby villages were evacuated on Wednesday evening as smoke billowed over the hills.