The Fire Service says fire has destroyed the upper floor of a Kilbirnie storage unit but the ground floor appears to have escaped with only water and smoke damage.

Firefighters have now begun to attack the fire from inside the building and expect this to take several hours as they work their way from unit to unit.

Assistant Area Commander Paul Smith says it has been a long and difficult fire to put out and it has been a hard slog for the firefighters that were called in from around the city to help.

"The fire is deep seated and we expect to be still fighting the fire tomorrow morning. Once it has been extinguished then a fire investigation will begin to determine the cause."

Earlier this afternoon a digger was brought in to pull off part of the roof and a side wall to cool down the fire and allow firefighters to direct foam inside.

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