Painted with red and black plant extracts, three of the isolated Indians observe the Brazilian-government helicopter in April 2010.

This specific tribe is not in imminent danger, but loggers in Peru are forcing other tribes into Brazil, where Indian reserves provide better protection, Meirelles said. (See "'Uncontacted' Tribes Fled Peru Logging, Arrows Suggest.")

"Some of the tribes in the region are historical enemies," he said. "We never know what might happen when they meet" and are forced to share land and food sources, he said.

There are three other tribes in the area, according to Meirelles. Across Brazil there are an estimated 70 isolated groups, although only 29 are confirmed, he said.

"There isn't a single group in the whole world nowadays who has not had any kind of contact with other people," he added. "They know we exist. They have just decided they don't want to make contact, and that's in their best interest."

The Acre-area tribes' first contacts with white men were in 1895, when rubber tappers arrived in this deep region of the Amazon rain forest, Meirelles said. "There have been reports depicting Indians using machetes from rubber tappers since 1905."

The latest pictures also show Indians wielding machetes and using metal pots.

(Related: "Photos Spur Debate on Protecting 'Uncontacted' Tribes.")

New Pictures Show "Uncontacted" Tribe "Well and Strong"

Last seen aiming arrows at an aircraft two years ago, an isolated Amazon group has now been photographed in unprecedented detail.