Two boys, aged 15 from Newport, have been arrested on suspicion of arson following a grass fire in the Gaer area of Newport last week.
They have since been released on police bail pending further enquiries.
Police are asking anyone with information on grass fires to come forward.
It comes as Crimstoppers announced a reward up of to £5,000 for information on deliberate grass fires across south Wales.
Deliberate grass fires not only blight the countryside, wildlife and the environment, they present a very real danger to members of the public. Whoever is responsible not only has to live with the consequences of the damage but the real possibility of causing injury to members of the public. They will also face prosecution which could result in a custodial sentence.
The total number of grass fires dealt with by South Wales Fire and Rescue Service are up by nearly 200% on last year, new figures show.
More than 100 blazes across Rhondda Cynon Taf and Merthyr Tydfil have been reported to police since the beginning of March.
People starting grass fires should be locked-up Wales' Public Services Minister will be told later today