The project from evangelical Christian group Ethnos360 comes after news that Brazil's far-right President Jair Bolsonaro has appointed a former missionary from the group to head the government's programs to protect uncontacted tribes.
Together, those developments are putting isolated indigenous peoples at risk of harmful contact with the outside world, including potentially devastating disease outbreaks, Survival International said.
"It's now clear that there's been a conscious decision by the Brazilian government to open up indigenous territories to evangelical missionaries, as a key step in the takeover of their lands and the exploitation of their gold, minerals, timber and other resources," said Sarah Shenker, campaign coordinator at Survival International.
"If it's not stopped, many tribes will be wiped out," she said in a statement.
The Brazilian Amazon is home to at least 100 isolated indigenous groups, more than anywhere in the world, according to Survival International.
Ethnos360, a Florida-based group formerly known as the New Tribes Mission, said on its fundraising site it had successfully raised the money to purchase an R66 helicopter to evangelize 10 previously unreachable groups in the rainforest in western Brazil.
In a fundraising video for the helicopter project, Ethnos360 pilot Jeremiah Diedrich described the region the group is targeting, near Brazil's border with Peru, as "the darkest, densest, hardest-to-reach place in all of South America."
The group, which believes in a literal interpretation of the Christian Bible, says its mission is to bring Christianity to all people whose "cultures and languages have isolated them from the gospel."
But the group "has a record of manhunts and contacts leading to death and disease, sex abuse in its schools, and bringing in deadly epidemics," said Survival International's director, Stephen Corry.
"These are the last people who should be anywhere near uncontacted tribes."
Survival International condemned the appointment last month of anthropologist and evangelical pastor Ricardo Lopes Dias to head the department on isolated tribes at FUNAI, the Brazilian government's agency for indigenous affairs.
Dias was a member of New Tribes Mission from 1997 to 2007.
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