A huge fire engulfing a swath of Brazilian jungle threatens the existence of remote indigenous tribes and may have been started by illegal loggers invading the territory, Greenpeace said Wednesday.

About 12,000 members of the Guajajara tribe and 80 people from the even more isolated Awa-Guaja live on lands affected by what the environmental group described as "one of the biggest forest fires ever registered within an indigenous territory in Brazil."

Already burning for two months, the blaze has consumed at least 45 percent of the 413,000- hectare (one million-acre) Arariboia Indigenous Reserve in Maranhao state, on the edge of the Amazon jungle, Greenpeace said.

The burned area is approximately equivalent to 190,000 football pitches. Aerial footage released by the group showed intense orange flames and columns of smoke erupting from multiple places in the jungle.

"It was shocking to see the gigantic dimensions of the destruction and to see that the Gaujajara and Awa-Guaja are the great victims of this tragedy," Greenpeace activist Danicley de Aguiar said.

"Beyond putting out the fire, the main worry is guaranteeing the survival of these peoples."

About 250 firefighters are attempting to control the weeks-long blaze, controlling a fiery frontline more than 100 kilometers (60 miles) long.

Brazilian media showed aircraft dumping chemical extinguishers over fires, but footage shot by Greenpeace in which a handful of men on foot squirted water from small hoses illustrated the authorities' inability so far to get to grips with the reported disaster.

- Battle for territory -

The area has been the scene of tensions between indigenous inhabitants and illegal Brazilian logging companies, with local tribes people attempting to patrol and defend their lands.

"Loggers are being accused of starting the fire as a reprisal," Greenpeace said. "Along with other indigenous territories in Maranhao, the Arariboia suffered invasion and systematic theft of wood."

Similar scenes have played out in other parts of Brazil where ancient tribes live in forests filled with hardwood trees sought the world over for construction and furniture.

"Illegal logging in indigenous lands is happening all over Brazil. And as indigenous peoples take measures to stop it, retaliation -- like violence or starting fires in the forest -- grows," Greenpeace said.

Indigenous leaders and environmental activists say that Brazil's aboriginal peoples now face another potentially devastating threat, but this time in Congress.

A law is under discussion that would give Congress, not the federal government, power to demarcate the borders of indigenous lands. On Tuesday, the draft cleared at committee level in the lower house and it is now ready for votes in the full chambers.

The law known as PEC 2015 is a constitutional amendment that could be crucial in the long-running struggle over control of huge areas of Latin America's biggest country.

Native peoples say PEC 2015 would deprive them of the relative objectivity of government bodies and put their fate in the hands of the notoriously corrupt legislature, where the powerful agricultural lobby could further weaken tribes' already shaky territorial rights.

At the opening of the Indigenous Games last week in the Amazonian city of Palmas protestors confronted President Dilma Rousseff over the law.

"You are killing our people!" one man yelled at the president before fleeing.