Firefighters tackled a large gorse blaze throughout the night near Llangollen.
North Wales Fire and Rescue Service received a call at 3.52pm on Sunday reporting a fire on Llantysilio Mountain.
Five fire crews – two from Wrexham and one each from from Ruthin, Bala and Llangollen – to the scene, as well as two off road vehicles, one from Llangollen and another from Abergele.
A spokesman for North Wales Fire and Rescue Service confirmed crews remained at the site throughout the night and were still in place on Monday morning.
The full extent of the damage is as yet unconfirmed but fire chiefs believe the cause could have been a controlled burn which then spread out of control.
A smaller fire in Llangollen had been reported to firefighters just minutes earlier at 3.48pm.
This incident caused damage to 100 square metres of gorse and was tackled by one crew from Llangollen, who used four beaters and said the incident had probably been caused by a hot ash from a steam train.
No injures were caused as a result of either incident.
There have been about 20 smaller incidents across North Wales involving grass, gorse and bracken since Friday, some deliberate, some as a result of controlled burning and others as a result of smoking materials or camp fires not being put out properly.
Stuart Millington, senior fire safety manager, said: “Whereas the number of incidents is relatively small, each has the potential to tie up resources and prevent us from attending other life threatening incidents.
“The dry weather has increased the risk of rural fires and we would like to thank those people notifying us of their controlled burning and who are acting in a safe and responsible manner.
“The controlled burning season comes to an end on March 31 and in the meantime we continue to urge people to inform us of any plans to undertake burns during the permissible burning periods.
“On the day of the intended burn we would ask that landowners inform fire service control on 01931 522 006 of the location of the burn. This will prevent time and resources being wasted by the fire service attending a controlled fire.
“During drier periods, fires involving grass, bracken and heather can develop very quickly, particularly in raised winds, resulting in fires getting out of control and spreading to neighbouring properties or forestry, with the need for the fire and rescue service to be called out to extinguish them.
“We are urging visitors to the countryside to take extra care when they are out and about and reduce the risk of fire.
“Make sure that any smoking materials are discarded and extinguished properly.
“If you are out camping, again make sure that campfires or barbecues are fully extinguished.”
See full story in the Free Press