Dozens of crews were scrambled to the blaze on the Rickarton Estate, near Stonehaven, in Aberdeenshire, yesterday afternoon.
Initially 50 firefighters tried to tackle the fire before reinforcements had to be called in from neighbouring stations as it spread to cover 10 hectares.
Crews fanned out across the area tackling large pockets of fire which were breaking out between pine trees throughout the afternoon.
Incident Commander Duncan Smith said last night: "Our crews continue to work tirelessly to prevent further fire spread from this incident.
"We are receiving excellent support from our colleagues at Police Scotland and from the Forestry Commission who manage the neighbouring Durris Forest. Firefighters are working on very challenging terrain."
The fire is understood to have started as a controlled burning which got out of control.
Firefighters used beaters and knapsack sprayers to try and extinguish the flames across the large expanse of land.