"The fire destroyed 21 hectares (50 acres) of the Deurnese Peel reserve and a 150 firefighters have been mobilised," spokesman for the eastern Brabant fire service Ton Driessen told AFP.
He added it was "a considerable area by Dutch standards".
The forested reserve, which is difficult to access, is in the southern Netherlands where the fire service announced a "code red" on Wednesday across seven provinces warning of a high fire risk after days of sizzling temperatures.
As in the rest of Europe sweltering in a summer heatwave "the high temperatures of the past few days and a shortage of water has left the vegetation fragile and vulnerable to flames," Driessen said.
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But he said the fire service was also uncertain about how the blaze started, and "suspicions" had been raised as in "an amazing coincidence" there had been a first fire a few hours earlier in another reserve about 20 kilometres (12 miles) from Deurnese Peel.
The blaze came six days after huge forest fires were sparked in Portugal over the weekend, which have killed more than 60 people and left 254 hurt.
Another blaze in a Dutch reserve on the northern island of Terschelling also erupted overnight, but was swiftly brought under control.
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