Fire crews across Wales say they are continuing to deal with "difficult conditions" as they tackle wildfires on hillsides and moors across the country.
South Wales Fire and Rescue said it had been inundated with a high volume of calls since Wednesday afternoon.
North Wales firefighters have also been trying to extinguish moorland fires on the Horseshoe Pass in Denbighshire.
The fire services said they would be working with police to identify those deliberating starting fires.
"Our fire crews, joint fire control, officers and specialist firefighting helicopter are working tirelessly," said one officer for South Wales Fire and Rescue.
"Deliberate wildfires have a significant impact on our resources and arson is a crime.
"We will be fully investigating these fires with our partners in South Wales and Gwent Police following completion of these incidents."
On Wednesday evening, the force had crews in at least five locations, including at Twmbarlwm Mountain, near Cwmcarn, and Fochrhiw in Caerphilly county, and at Blaenllechau, Cefnpennar and Maerdy Mountain in Rhondda Cynon Taff.
Crews from North Wales Fire and Rescue are continuing to tackle gorse fires on Llantysilio mountain, on the Denbighshire moors between Llangollen and Wrexham.
The south Wales firefighters say they have the resources to tackle the ongoing blazes, and have asked the public not to keep calling 999 to report them.
"We'd like to thank local communities for their awareness and for bringing these incidents to our attention, we do have resources at all of these incidents now and would kindly ask that the local residents no longer call 999 to alert us to fires at these locations," said a fire official,