Exodus as southern Chile's Puyehue volcano erupts for the first time in half a century, spewing ash up to 10 kilometres.

A volcano dormant for decades has erupted in south-central Chile, spewing ash to the height of 10 kilometres into the sky, and prompting the government to evacuate several thousand residents, authorities said. Winds spread the ash towards neighbouring Argentina, darkening the sky in the ski resort city of San Carlos de Bariloche, a government official there said on Saturday, adding the city's airport had been closed.

The eruption in the Puyehue-Cordon Caulle volcanic chain, about 920km south of the capital, Santiago, also prompted Chilean authorities to shut a heavily travelled border crossing into Argentina.

'Constant seismic activity'

It was not immediately clear which of the chain's four volcanoes had erupted because of ash cover and weather conditions. The chain last saw a major eruption in 1960. Local media said the smell of sulfur hung in the air and there was constant seismic activity.

"The Cordon Caulle [volcanic range] has entered an eruptive process, with an explosion resulting in a 10-kilometre-high gas column," ONEMI, the state emergency office, said. The government said it was evacuating 3,500 people from the surrounding area as a precaution.

It was the latest in a series of volcanic eruptions in Chile in recent years. Chile's Chaiten volcano erupted spectacularly in 2008 for the first time in thousands of years, spewing molten rock and a vast cloud of ash that reached the stratosphere. The ash also swelled a nearby river and ravaged a nearby town of the same name.

The ash cloud from Chaiten coated towns in Argentina and was visible from space. Chile's Llaima volcano, one of South America's most active, erupted in 2008 and 2009.

Chile's chain of about 2,000 volcanoes is the world's second largest after Indonesia. About 50 to 60 are on record as having erupted, and 500 are potentially active.