Firefighters tackling grassland blazes claim they have been left stretched by "complacent" bosses.
Two major fires in Guisborough Woods and the Eston Hills in East Cleveland have been raging for the past 10 days.
The Fire Brigades Union (FBU) said chiefs left crews "crying out for support" and failed to call in standby firefighters due to recent cuts.
Cleveland Fire Brigade accused the union of "scaremongering" and said there was no need for extra staff.
The union said crews on the scene were stretched to the limit, while available back-up crews were left "waiting by the phone".
FBU Cleveland secretary Davy Howe said: "Up to 30 firefighters have been battling difficult conditions and standing by at the scene of fires for up to 20 hours over the last seven days.
"The complacent attitude of the brigade is astounding. Crews were crying out for support, while management gambled and left willing and able firefighters at home.
"Additional firefighters at the scene would undoubtedly have been safer and more effective in tackling the huge fires."
Mr Howe said the situation was a direct result of a recent decision to cut the number of fire engines.
In response, chief fire officer Ian Hayton said he was "disappointed" the union was engaging in "scaremongering which unnecessarily alarms the public".
He said the service had followed its agreed plan for such a scenario, adding: "We did not need to supplement our resources as through effective management of our staff, fire appliances and specialist off-road vehicles, we were able to extinguish forest and grass land fires and still have appliances available to deal with any other emergency incidents."