A thick cloud of ash from the erupting Chaiten volcano in southern Chile spread across a swath of South America on Wednesday, prompting fears of a health crisis for the people caught in its wake.

The enormous slow-moving plume spread thinly over Argentina's ski resort of Bariloche, 230 kilometres northeast of the volcano, and ash particles were detected as far away as Mar del Plata, 1600 kilometers to the east.

Authorities are fearful that the airborne ash particles might reach Buenos Aires and its surroundings, raising health concerns for some eight million people.

Thousands of Chileans have already fled their homes after the volcano, located 1300 kilometres south of Santiago near Chile's border with Argentina, erupted violently on Tuesday, spewing ash and lava over its surroundings.

"Everything depends on which way the wind blows in the next few hours," said a spokesperson for Argentina's meteorological service.

He said forecasters were fairly certain that the skies of more Argentine towns would be darkened by the volcano's after-effects, adding: "We don't see any prospect of a change in the weather."

Chile's National Emergency Office said the area around a 30-kilometer radius of the volcano had been completely evacuated.

The 4000 residents of Chaiten, the town nearest to the volcano whose streets were covered by a thick layer of ash, were evacuated on Tuesday. Only a handful of emergency personnel and news reporters remained.

Many of the town's businesses relocated to the city of Puerto Montt, some 200 kilometres to the south, and residents also took shelter there.

"I didn't want to leave, but there was too much noise, and ash fell all day long," said Rosa Mayorga (62).

Another 1000 people were forced to relocate from the town of Futaleufu, while officials in neighbouring areas distributed face masks and drinking water.

The dense plume of volcanic ash took a northeasterly turn overnight to Wednesday after moving for days in a southeasterly direction, the head of the National Emergency Office, Rodrigo Rojas, told AFP.

Some of Mar del Plata's 600 000 inhabitants reported seeing ash particles in the air and Argentina's National Weather Service said, "it all depends on how the winds will blow in the next few hours whether the ashes will reach the capital (Buenos Aires)."

Otherwise "there have been no reports of significant changes in seismic activity or emission of ash" since the volcano began to erupt violently on Tuesday, Rojas said.

Three leading airlines — Aerolinas Argentinas, Austral and Chile's LAN — cancelled flights across the region, including flights to the Atlantic port cities of Bahia Blanca, 900 kilometres northeast of the volcano, and Comodoro Rivadavia, located 575 kilometres southeast.

"The airports are open but the airlines cancelled flights because the volcanic ash gets in the airplane turbines and could cause material damage," said an aviation official at the Jorge Newbery airport north of Buenos Aires.

The official said it was likely that flights would remain grounded Thursday given the weather forecast.

Officials in the Argentine province of Chubut, east of the eruption, said some 160 schools that had been closed since Friday due to the volcanic ash had now reopened.

Town authorities in parts of Chubut have distributed drinking water and masks to their inhabitants.

A Chilean vulcanologist warned on Tuesday that the eruption was only in the early stages, and that a larger explosion was possible.

"There could be a major explosion that could collapse the volcano's cone," said Luis Lara of the National Geologic and Mining Service.

AFP