A decree issued by President Jair Bolsonaro authorises the deployment of soldiers in nature reserves, indigenous lands and border areas in the region.
The announcement comes after intense pressure from European leaders.
Mr Bolsonaro has criticised the reaction of other nations, insisting wildfires “cannot be used as a pretext” for punitive sanctions.
It came after France and Ireland said they would not ratify a huge trade deal with South American nations unless Brazil does more to tackle blazes in the Amazon.
Many of the fires are thought to have been started deliberately, with suspicion falling on farmers who may benefit by having more available land.
Mr Bolsonaro has declared support for the clearing of areas of the Amazon for agriculture or mining and he has faced heavy criticism from experts and campaigners who say his administration has given a green light to rainforest destruction.
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