Suspected arsonists climbed over barbed wire fence to start a blaze in a factory yard.
Firefighters were called to Feldon Mill in Kirkham Lane, Heaton at 5.15pm on Saturday.
Crews had to scale the surrounding walls, which were topped with barbed wire, to douse the flames, which damaged a trailer and a skip.
Crew commander Mike O'Neil said: "We suspect it was arson, but whoever started it must have had to climb the walls and get past the barbed wire before lighting the fire."
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