Fire crews have been battling a mountain fire in Denbighshire since Friday evening.
The blaze broke out among the gorse and bracken at Llangollen at about 22:00 GMT and affected 50,000 sq metres.
It is being allowed to burn out under surveillance, with one appliance remaining at the scene.
Deliberate mountain fires have a "hugely detrimental" impact on wildlife and the environment, North Wales Fire and Rescue Service said.
Ass Ch Fire Officer Richard Fairhead said: "I urge people to really stop and think about the consequences of gorse fires.
"As we have seen in the past, such fires place tremendous pressure on resources, with our crews being committed for long periods of time trying to bring them under control, which in turn delays firefighters from attending life-threatening incidents elsewhere.
"We will not tolerate deliberate grass and mountain fires - not only do they lead to the destruction of the mountainside and wildlife, but they also put lives at risk whilst our fire crews are dealing with needlessly set fires."
He added fires involving grass can develop extremely quickly in dry conditions and raised winds, and can spread to neighbouring properties or forestry.