Volcanic ash raining down from the Chilean volcano Chaiten may cause long-term environmental damage and harm the health of people and animals in Patagonia, scientists say.

Ash from the volcano, which started erupting 10 days ago for the first time in thousands of years, is made up of pulverised rock containing all kinds of minerals.

It has spoiled lakes, rivers and lagoons, coated plants in a dense layer of gray, and altered the sensitive habitat of animals now struggling to survive. Satellite images show a white stripe smeared across the southern part of South America.

Though it is too early to say what the long-term effects will be, ecologists say life has permanently changed in the region's pine and cypress forests, inhabited by pumas and huemules, a rare species of deer.

"I am tremendously worried because this is an environmental, social and ecological disaster," said Alejandro Beletzky, an environmental scientist in a soot-covered swath of Argentina.

"The presence of volcanic ash in the region, which falls constantly, is very risky for humans, plants and animals," he said near Esquel, a town 2000 kilometres southwest of Buenos Aires.

Government officials have insisted the ash is not toxic, though people in the Argentine provinces of Chubut and Rio Negro, and Chile's Tenth Region have complained of burning eyes, breathing trouble and tainted water.

The volcanic ash blowing east across the Andes mountains from Chile has dusted hundreds of square miles of Argentina. Nearby airports have closed because of poor visibility and worries the rocky ash could damage jet engines.

Chile's chain of volcanoes, the second-largest in the world, includes some 2000 of which 500 are potentially active. Chaiten sits 1220 kilometres south of the capital Santiago.

On both sides of the border, pastures were blanketed in ash, a few animals tried to eat grass, and birds perched on trees looked like concrete statues.

"We don't think the ash is toxic, but we need to take into account the long-term effects on the digestive and respiratory systems of animals," said Christian Hepp, an agronomist for Chile's national institute of livestock studies, which is testing the soil of cow and sheep pastures clouded by ash.

In Chile, evacuated residents complained of being thrown into a state of limbo, not knowing when, or if, they would be able to return. Chaiten has shot a towering plume of ash 20 kilometres into the sky, forcing thousands of people to evacuate within a 50 kilometres radius.

The column might descend gradually. But in a worst-case scenario, ash and molten rock would drop quickly and engulf the town of Chaiten, just 10 kilometres from the volcano, killing everything in its path.

"We can't put anybody's life at risk," President Michelle Bachelet told weary evacuees huddled in shelters.