Its members belong to what is thought to be the oldest surviving culture in the Andes, a tribe that has survived for 4,000 years on the barren plains of the Bolivian interior. But the Uru Chipaya, who outlasted the Inca empire and survived the Spanish conquest, are warning that they now face extinction through climate change.

The tribal chief, 62-year-old Felix Quispe, 62, says the river that has sustained them for millennia is drying up. His people cannot cope with the dramatic reduction in the Lauca, which has dwindled in recent decades amid erratic rainfall that has turned crops to dust and livestock to skin and bones.

"Over here used to be all water," he said, gesturing across an arid plain. "There were ducks, crabs, reeds growing in the water. I remember that. What are we going to do? We are water people."

The Uru Chipaya, who according to mythological origin are "water beings" rather than human beings, could soon be forced to abandon their settlements and go to the cities of Bolivia and Chile, said Quispe. "There is no pasture for animals, no rainfall. Nothing. Drought."

The tribe is renowned for surviving on the fringe of a salt desert, a harsh and eerie landscape which even the Incas avoided, by flushing the soil with river water. As the Lauca has dried, many members of the Uru Chipaya have migrated, leaving fewer than 2,000 in the village of Santa Ana and the surrounding settlements.

"We have nothing to eat. That's why our children are all leaving," said Vicenta Condori, 52, dressed in traditional skirt and shawl. She has two children in Chile.

Some members of the tribe blame the crisis on neglect of the deities. The chief has lobbied for greater offerings and adherence to traditional customs. "This is in our own hands," he said.

Scientists say rising temperatures have accelerated the retreat of Andean glaciers throughout Bolivia, Colombia, Ecuador and Peru. A ski resort in Bolivia's capital, La Paz, the highest in South America, closed several years ago because of the retreat of the Chacaltaya glacier. The Intergovernmental Panel on Climate Change warned in 2007 that warmer temperatures could melt all Latin America's glaciers within 15 years. A recent World Bank study sounded fresh alarm on the issue.

Indigenous groups from around the world are meeting in Alaska this week to discuss global warming. "Indigenous peoples are on the frontlines of climate change," said the host, the Inuit Circumpolar Council. A new Oxfam report, meanwhile, has warned that within six years the number of people affected by climate-related crises will jump by 54% to 375 million.

Evo Morales, Bolivia's president, told the Guardian that his government would form a united front with indigenous groups for a "big mobilisation" at a summit in Denmark this year to draw up a successor to the Kyoto treaty. They intend to push industrialised countries to cut carbon emissions. "We are preparing a team from the water and environment ministries to focus not only on the summit but beyond that."

One of South America's poorest countries, Bolivia is struggling with competition for natural resources. Water scarcity has hit La Paz and its satellite city, El Alto, prompting conservation campaigns. The shortage is nationwide. The Uru Chipaya accuse Aymara communities, living upriver from the Lauca, of diverting more and more water supplies. "It's a dual cause: climate change and greater competition. The result is an extremely grave threat to this culture. I am very worried," said Alvaro Díez Astete, an anthropologist who has written a book on the tribe.

With so many of the young people migrating to cities, where they speak Spanish, the Uru language could disappear within a few generations. Some Uru Chipaya fear the battle for cultural survival could already be lost. The rutted streets of Santa Ana are largely deserted and little disturbs the stillness of the dry plains that once were fields.

Several dozen, mostly elderly, people gathered on a recent Sunday to share soup from communal pots. "We are at risk of extinction," said Juan Condori, 55. "The Chipaya could cease to exist within the next 50 years. The most important thing is water. If there is no water the Chipaya have no life."