The Fire and Rescue Service said it believes a large gorse fire in the Mournes was started deliberately.
Most of the fire crews tackling the blaze in the Dinny Water region near Annalong have withdrawn.
Police have warned they will take serious action against anyone caught starting fires.
Fire officer Brian Irvine said time spent fighting deliberate fires means crews cannot respond to other emergencies.
"I would say to those people who are lighting the fires, that they should really think about if there's someone who is near to you, could be a loved one, family member or a friend.
"If they're involved in a road traffic collision or if there's a house fire or an accident at industrial premises.
"If you require the Fire Service, we cannot be there as quickly as we normally would be, because we are being tied up at these wild fires."