Red-painted Indians from an unknown Amazon jungle tribe brandished bows and arrows as they were photographed from a passing helicopter.

Brazilian officials discovered the tribesmen earlier this month in Acre State, near the country's border with Peru, on a trip designed to highlight the threat posed to remote peoples by deforestation.

One of the photographs they took showed two indigenous men in red trousers standing in a tiny clearing outside their huts, their bows aloft, while a third, far darker man stands watching, a white cloth tied around his waist.

In another photo, around a dozen Indians stand outside their huts, also preparing to fire arrows, while one woman appears to be standing just inside a doorway, a baby in her arms. Some of the Indians appear to be wearing headdresses.

Other pictures show the tent-shaped dwellings that line the small clearing, surrounded only by dense jungle.

The tribesmen were discovered by an expedition organised by Funai, the Brazilian government's Indian affairs department.

Funai said it does not know to which tribe they belong but there are said to be more than 100 "uncontacted" tribes around the world, half of them in either Brazil or Peru.

"We did the overflight to show their houses, to show they are there, to show they exist," José Carlos dos Reis Meirelles Júnior, an uncontacted tribes expert who works for Funai, said. "This is very important because there are some who doubt their existence."

In a statement released through the group Survival International, he added: "What is happening in this region is a monumental crime against the natural world, the tribes, the fauna and is further testimony to the complete irrationality with which we, the ’civilised’ ones, treat the world."

Surivival International says that uncontacted tribes face a range of dangers, including being forced off their land and killed or ravaged by new diseases. It has launched an urgent campaign to get tribal lands protected.

"The world needs to wake up to this, and ensure that their territory is protected in accordance with international law," said Stephen Corry, director of Survival International. "Otherwise they will be made extinct."

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