The Indians are believed to have fled from Peru into Brazil because of illegal loggers sweeping through Peru's rainforests in search of rare mahogany, known as 'red gold.'
The loggers are destroying the Indians' territories, forcing them to seek refuge elsewhere and leading to dangerous contacts with outsiders.
The Indians suddenly appeared at a village called Bananeira and spent a day and a night there. Another, smaller group of uncontacted Indians was also spotted at a nearby settlement called Liberdade.
"We are on the verge of disaster. Illegal logging in protected areas in Peru is pushing the uncontacted tribes into Brazil, which could cause conflicts and lead to their appearance in places where they have never been seen before," said José Carlos dos Reis Meirelles Júnior, head of the Indian Protection post near the Peru border.
Meirelles made his statement in an urgent alert sent to the Brazilian government.
Peru has some of the world's last commercially-viable mahogany stands, growing in areas inhabited by some of the world's last uncontacted tribes. Because of their isolation, the tribes do not have immunity to outsiders' diseases and any form of contact with them can be fatal.
Stephen Corry, director of Survival International, an organization in defense of tribal peoples, commented: "If it's not 'black gold', it's 'red gold.' The Peruvian government must act now to stop the logging on the uncontacted tribes' land. If it doesn't, they could be the first people to be made extinct in the 21st century."