Firefighters are monitoring a large grass fire on a mountain in Tonypandy, Rhondda Cynon Taf.
Crews have been called three times since Saturday night but cannot access the blaze because it is on a steep cliff face.
A spokeswoman for South Wales Fire and Rescue Service said: "It's too dangerous for the crews.
"They cannot access the fire and are just there watching it as they allow it to burn."
She added: "They are waiting for it to burn out but it will probably burn for quite some time."
She said the fire service has had over 20 calls about the fire, which is near Edmoundstown Road.
Steve Doel, Tonypandy fire station manager, said: "We're monitoring the fire but there is no immediate danger of it spreading to property."
Crews from Porth and Tonypandy have been at the scene at various times throughout the day, along with a 4x4 vehicle.
They were first called at 2200 BST on Saturday, then went back to the scene at 0850 BST on Sunday and again on Sunday afternoon.