The spokesman said the fire was being treated as suspicious and a specialist fire investigator was on the scene.
Daryl Muir runs a business next door and said the vehicles were large truck and trailer units.
The yard was used by independent transport companies to store their vehicles, he said.
"I could see the flames and smoke breaking though the tops of the trucks," he said.
Muir had feared his neighbouring building would also catch alight but thanks to the quick response of firefighters the blaze was contained before it did so.
Police have been notified and a spokesman said they were supporting the Fire Service.