By Terry Wade and Marco Aquino

LIMA, Sept 28 (Reuters) - Ecologists have photographed a little-known nomadic tribe deep in Peru's Amazon, a sighting that could intensify debate about the presence of isolated Indians as oil firms line up to explore the jungle.

Carrying arrows and living in palm-leaf huts on the banks of the Las Piedras river, the tribe was glimpsed last week by researchers flying over the Alto Purus national park near the Brazilian border to look for illegal loggers.

"We saw them by chance. There were three huts and about 21 Indians -- children, women and young people," said Ricardo Hon, a forest scientist at the National Institute of Natural Resources.

Hon said an indigenous group using the same kind of huts was seen in the region in the 1980s, and advocacy groups said they appeared to be part of the Mascho Piro tribe.

The sighting of the indigenous group comes as Peru's government is encouraging foreign companies to look for oil in the rainforest.

Environmental and Indian rights groups firmly oppose the exploration in the remote jungle area about 550 miles (900 km) east of Lima, the South American country's coastal capital.

Indigenous people who have shunned contact with the rest of society are believed to live within some of the dozens of parcels being auctioned across the country for petroleum prospecting, some of them in the Amazon.

"The Peruvian government is actively promoting oil and gas exploration in areas where uncontacted tribes live," said David Hill, a researcher with London-based advocacy group Survival International.

The organization estimates that up to 15 isolated tribes live in Peru, the most after Brazil and Papua in Indonesia.

Peru's state oil company PetroPeru says it considers tribes that shy away from outsiders to be safe as they live in protected reserve areas, which are excluded from petroleum auctions. But its president, Daniel Saba, was criticized earlier this year for saying the notion of hidden tribes was "absurd."

Rights groups say nomadic tribes travel in and out of national parks depending on the season, and encroaching loggers or oil company workers could expose them to deadly diseases. In the past, disease halved many Amazon tribal populations.

Documenting how many isolated groups exist is notoriously difficult because some tribes have hidden deeper in the forest or moved to new locations after brief encounters with outsiders.

"Quite a few groups have probably made a conscious choice to retreat and not build long-term relations with newcomers," said Suzanne Oakdale, an anthropology professor at the University of New Mexico.

"Often, 'uncontacted tribes' means uncontacted by a government institution, but the groups have long and complicated histories with other people," she said.

AlertNet news is provided by