The worst land grab in decades in the Brazilian Amazon is threatening the survival of isolated tribes that have no contact with the outside world, a rights group said Wednesday.

Ranchers and settlers in the remotest reaches of northwestern Brazil are voraciously cutting down rainforest to farm crops, encroaching on the ancestral lands of three uncontacted groups, said Survival International.

The land grab is also threatening another tribe, the Uru Eu Wau Wau, or "Harpy Eagle" people, that has only limited contact with the outside world, said the London-based group.

Warning the groups face "annihilation," it accused local politicians in the state of Rondonia of backing the deforestation, even though the area is officially designated as an indigenous reserve and sits within a national park, Pacaas Novas.

Because isolated peoples' immune systems have never been exposed to the outside world's diseases, the land grab risks causing devastating outbreaks, Survival said.

"Around the world, industrialized society is stealing tribal lands in the pursuit of profit. What's happening in Brazil is simply a continuation of the invasion and genocide which characterized the European colonization of the Americas," said the group's director, Stephen Corry.

The organization quoted a letter the Harpy Eagle tribe sent to Brazilian police, in which they call the land grab "extremely serious."

"We are very worried because the invasions are close to our villages and putting the lives of women, old people, children and men at risk," said the letter.

Experts estimate between 50 and 90 percent of the populations of Brazil's isolated tribes were wiped out when the government initiated contact with them in the 1970s and 80s -- official policy at the time.

Today, the government tries to avoid any contact with isolated peoples, in order to protect them.

Brazil is home to some 900,000 indigenous people from 305 different ethnic groups.