About 50 firefighters are tackling a blaze at an industrial unit containing hazardous materials after explosions were heard.
The fire on Parham Airfield, near Framlingham, Suffolk, is understood to be at a vehicle components factory and broke out at about 09:45 BST.
A witness reported hearing "loud bangs and explosions", and plumes of black smoke can be seen for miles.
The fire service said no-one had been injured in the incident.
Richard Lapage, who was driving "a couple of hundred metres away", stopped his car when he heard "loud bangs and explosions".
"It's at Hatcher Components, a fibreglass place," he said.
Suffolk Fire and Rescue Service said 13 crews were tackling the "hazardous materials" fire and have urged residents and nearby businesses to keep doors and windows closed.
Group commander Paul Goodman said: "There were a few loud bangs and those sounded a lot worse than it was.
"They were actually empty chemical drums but once they start heating up there's always something inside and the tops tend to blow off - but they go off quite spectacularly - and then they can travel for quite some distance as well."
The fire service has scaled back crews at the scene, and shortly after 14:00 BST said six remained on site.
Suffolk Police confirmed road closures were in place in the area.