Funai, Brazil's agency on indigenous affairs, responsible for protecting the tribes, says the tribe members had been collecting eggs along the river when the miners approached them and killed them, the New York Times reports.
Federal prosecutors confirm that they have opened an investigation, but haven't given any details. The Javari Valley, the area of the alleged killings, is known as the Uncontacted Frontier, and includes more uncontacted tribes than anywhere on earth, Survival Unlimited said.
"It was crude bar talk", a spokesperson for Funai [the National Indian Foundation in Brazil], told the NY times. "They even bragged about cutting up the bodies and throwing them in the river".
Prosecutor Pablo Luz de Beltrand, who is in charge of the case, said that the investigation had started, but he could not delve into the information.
Fumai said the miners allegedly displayed a hand-carved wooden paddle as a trophy of the killings. At other bases, they cut staffing.
"We give tribal peoples with experience of contact a platform to speak to the world, and raise awareness of this urgent and horrific humanitarian crisis".
Latest updates from around the web as Irma heads toward Tampa
We won't have hurricane force winds in the state, but tropical storm force winds are possible for parts of the state. Sunday as forecasts are locking in on a path that includes the Keys, southwestern Florida and the Tampa Bay region.
If 10 indigenous tribespeople are confirmed to have been killed, that could represent 20 per cent of the tribe, the group says.
Survival International believes this is the culmination of the Brazilian government's decrease in funding for teams who look after uncontacted indigenous groups.
Prosecutors are also investigating another complaint about the alleged killing of indigenous people from the isolated Warikama Djapar tribe.
Uncontacted Indians in the Brazilian Amazon, filmed from the air in 2010. The attack is thought to have occurred in May but has not been confirmed. Survival International portrayed Temer's legislature as "savagely against Indian, and has close connections to the nation's capable and hostile to indigenous agribusiness campaign".
Survival director Stephen Corry said Brazil's president Michel Termer and his government bore "a heavy responsibility for this genocidal attack" if the reports were true.
This is not the first time that indigenous peoples' lives been threatened.