Fire crews have been tackling blazes across the south of the country including Machen mountain near Caerffili and Aberpennar in Rhondda Cynon Taf.
Gwent Police say that a number of deliberate grass fires have been started in the past week in Blaenau Gwent, Caerffili, and Torfaen.
Gwent Police's Rural Crime Team said that they had been assisting South Wales Fire and Rescue Service by monitoring ablaze areas using drone technology.
Drones have also been used to undertake more regular patrols of the countryside.
South Wales Fire and Rescue issued a warning on social media calling for the criminal behaviour to stop.
They said that they were still tackling a number of grass fires across the area on Monday morning (26 April).
Larger blazes had broken out near Aberpennar and Machen.
They said that fire crews had attended nearly eighty suspected deliberate grass fires over the weekend.
They also urged members of the public to report any incidents, or any suspects, to the police.
South Wales Fire and Rescue said: "What may seem like a little bit of fun can spread quickly and cause damage to property, endanger the environment and put lives at risk."