The incident happened during the early hours of this morning on a farm in North Wales.
North Wales Fire and Rescue Service said the structure contained livestock, hay and fertilizer.
A spokesman for the fire service said the cause of the blaze is still under investigation.
The police helicopter was scrambled in a bid to catch the suspects.
North Wales Police said: “We are currently dealing with a report of an arson in Holywell and have deployed the helicopter to assist with the incident.”
Two appliances remain at the scene this morning damping down the smouldering wreckage, reports the Daily Post.
Sgt Alison Sharp of North Wales Police said: “This has been a devastating fire in which animals have been killed and which will have an impact on the livelihood of the farmer.
“We are treating this incident as arson and I would appeal to anyone who has information to contact police on 101 quoting reference W012414."
Last year, the Country Land and Business Association (CLA) urged farmers to take precautionary measures in order to reduce their chances of being a victim of arsonists.
CLA East Regional Director Ben Underwood said: “Deliberate straw stack fires destroy important material used in arable and livestock farming, as well as a valuable renewable energy source – it’s not just a by-product.
“These fires can spread rapidly threatening buildings, livestock, machinery, and potentially human lives.
“Deliberate fire setting causes untold problems and the people involved do not give any thought to the consequences."