The fires often happen in areas where access is difficult and water supply is limited
Fire crews have been called to over 100 grass fires across Wales since Friday leading to warnings cover is being stretched to the limit.
Some of the larger fires have been in Rhondda Cynon Taf, Bangor, Gwynedd and on Gower, Swansea.
The three Welsh fire services say some have been started deliberately, either maliciously or by controlled burning by land owners quickly spreading.
People have been warned their lives and those of firefighters are at risk.
On Monday evening South Wales Fire and Rescue Service said crews were "incredibly busy" tackling a series of grass fires in the Rhondda area.
At the same time their colleagues in north Wales were dealing with a large fire on Bangor Mountain and at Mynytho near Pwllheli.
In one of the most serious incidents in recent days firefighters from four stations spent around 12 hours dealing with a grass fire that spread over approximately 100 hectares at Rhosilli Hill on Gower.
Explain to children the dangers of playing with and lighting fires
Dispose of smoking materials such as cigarettes safely
Clear away bottles, glasses and broken glass to avoid them magnifying the sun
Never leave camp fires or barbecues unattended and extinguish properly after use
Don't attempt to put out grass fires yourself as they can travel very quickly and change direction without warning
Report anyone seen deliberately lighting fires, or suspected of starting fires, to Crimestoppers on 0800 555111
Source: Mid & West Wales Fire and Rescue Service
Dean Morgan, community safety manager for the Mid and West Wales Service, said the blaze on Sunday was made all the more difficult as crews had to get to the fire on foot due to the terrain.
"Grass fires can have very serious consequences, these sort of fires are extremely dangerous and unpredictable, and they can quickly spread out of control," he said.
"We have experienced a rise in grass and forestry fires in the past few weeks.
"Deliberate fire setting presents a serious risk of injury and death to the fire setter and also the firefighters who attend the incident to extinguish the fire.
"These fires prove a huge drain on resources and firefighters tackling these fires may be delayed in attending genuine emergencies such as people trapped in house fires or road accidents."
The North Wales Fire Service said one of the biggest problems was landowners attempting controlled burning.
'Gorse and bracken'
A spokeswoman said: "Many incidents occur as a result of controlled burning getting out of control and affecting large areas of gorse and bracken.
"Quite often these fires happen in areas where access is extremely difficult and water supply is limited.
"Fires like this place tremendous pressure on resources, with firefighters often tied up for a considerable length of time trying to bring these fires under control."
It has urged landowners to let the service control room know about any controlled burning prior to it starting so resources are not wasted attending a controlled fire.