Brazil's Funai agency, which handles indigenous affairs, said that the killings were carried out by the Yanomami tribe in the Amazonian state of Roraima on the frontier with Venezuela.
"The deaths were reported to us by the Yanomami people's association and a team from Funai is preparing to travel there," a spokesman told AFP.
A police spokesman for Roraima state told AFP that "the causes of the conflict are not known."
The Yanomami are one of the Amazon's largest relatively isolated tribes, with an estimated population of about 35,000, according to advocacy group Survival International.
Their pristine ancestral lands have been steadily encroached upon by illegal gold miners who are blamed for introducing new diseases and polluting the rivers and forest. The Yanomami are also under pressure from cattle ranchers on the fringes of their rainforest territory.
Police said they would accompany Funai to the area, probably next week, and that the killings occurred at the start of the month but had only been confirmed now.
"We don't know the motive for the conflict because the tribespeople from the village where the deaths occurred told us by radio that they will not explain what happened until we personally go there," Junior Hekuari, chairman of the Yanomami association, told G1 news site.
