Environmental authorities have declared a "red alert" as fires raged towards one of Brazil's biggest and best preserved Indian reservations.
Flames were devouring farmland in central Mato Grosso state and heading for the Xingu National Park, where 17 different indigenous groups live in a densely forested area bigger than Belgium and prized by scientists for its rich biodiversity.
A spokesman for the government's National Indian Foundation said the flames were just 2.5 miles (4km) from the park's perimeter.
But the head of the national Environment Institute (IBAMA) estimated from satellite images that the main focal point of the fire was still 25 miles (40km) away, in the farming region around the town of Sao Jose do Xingu.
"We have the ability to deal with this fire before it reaches the Indian area," IBAMA president, Mr Eduardo Martins, said.
"This kind of forest is dry and would burn easily, which is why it must be stopped."
He said the Capoto/Jarina Indian Reservation, located directly north of the Xingu park, was also in danger.
IBAMA put a newly created rapid response plan into action for the first time on Monday, sending 30 firemen equipped with water tanks and rubber mats from the Mato Grosso capital, Cuiaba, on an air force plane to the burning area.