South Wales Fire Service said it had seen a 52% increase in the number of deliberate grass fires in the last eight weeks compared with the same period last year.
It said it had 248 calls in eight weeks this year, including one weekend where 70 deliberate wildfires were started.
In a statement, the brigade said: “Wildfires are extremely dangerous and can spread extremely quickly, putting lives in danger, causing significant damage to property and the environment including causing harm to wildlife.”
Mid and West Wales Fire Service also reported a 24% increase in grass fires this year compared to 2019, with more than 600 call outs so far.
North Wales Fire Service, which refused to give BBC Wales figures for the number of grass fires it has been called to this year, said it has had an increase in the number of fires – many caused by improper use of barbecues and camp fires.
Paul Scott, senior fire safety manager for the service, said: “The dry weather we have experienced has meant we have been called out to a number of grass and gorse fires, tying up crews for hours at a time when we are looking to reduce demand on our firefighters.”
Grass fires, whether deliberate or accidental, have led to increasingly negative impacts on the wildlife, as well as the air and water quality.
NRW is urging people to be responsible as more trips are made to the countryside.
Richard Owens, from NRW, said: “People don’t realise how easy it is for a rogue ember from a barbecue or bonfire, or discarding of a cigarette that is still lit, to develop into something bigger.”