Almost 60 firefighter were initially tasked to a large blaze outside Portadown on Saturday night.
The fire began at business premises along the Blackisland Road in Annaghmore shortly before 10pm.
The inferno, which raged into the night sky, saw 55 firefighters and six appliances from Portadown and nearby stations attend.
Four appliances remain at the scene as of Sunday morning.
It is understood the bulk of the fire was centred on large stack of hay bales. Fire crews battled intense heat to keep nearby buildings safe.
NIFRS Group Commander Mark Smith, speaking to BBC Radio Ulster, said: “It was a very large fire and still is and continues to be because it’s a large quantity of hay on fire and this takes a long time just to extinguish.
“We will do an investigation after the fire is completely out”.
Police in Portadown confirmed the report of a large fire at an industrial complex at Derrycorry North at approximately 10.20pm.
Inspector McCullough said: “Our officers, along with the Northern Ireland Fire and Rescue Service attended. It is our understanding that a large amount of hay bales sparked ignition resulting in the blaze.
“A substantial amount of monetary damage has been caused. Our colleagues from the Fire Service remain at the scene and are expected to as they continue to control the fire. There are no further details at this time.”
The road has been closed and motorists are advised to seek an alternative route.