A violent storm described as a freak "tornado" shredded hundreds of houses and killed at least 16 people in the southern part of South America on Tuesday, officials said.

Northern Argentina and southern Brazil, and the small countries of Uruguay and Paraguay wedged between them, were hit by a fierce atmospheric mass packing rain, hail and winds over 120 kilometers (70 miles) per hour.

In northeastern Argentina, 10 people died, including seven children, authorities said.

More than 50 others were injured, and trees and power lines were toppled in the towns of Santa Rosa, Tobuna and Pozo Azul, said Ricardo Veselka Corrales, head of the local civil defense office.

Witnesses and local media described the storm as a tornado.

Meteorologists were wary, although the US National Climatic Data Center said the affected zone is the only place in South America with a likelihood of experiencing the high-speed spinning tubes of destructive wind.

"It could have been a tornado," said Jorge Leguizamon, of Argentina's National Meteorological Service.

"The phenomenon still hasn't been classified. Experts will have to evaluate the damage," he said.

What was clear was that "it's not normal for this area," said the provincial minister, Daniel Franco.

"We've always had very strong winds and torrential rains here. But this was a phenomenon never seen before. Houses were completely destroyed," he said.

The devastation was "incredible," said the mayor of San Pedro, Orlando Wolfart, noting that several homes had been wiped from their foundations.

Television images showed a destroyed landscape, with several homes leveled and others still standing but with their roofs ripped off.

In the southern Brazilian state of Santa Catarina, similar devastation occurred from what the region's civil defense service agreed was "a probable tornado."

Four people died in that state when winds ravaged 37 towns and villages, knocking over more than 100 homes and blasting others with hail big enough to puncture roofs, it said in a statement.

Another two people died in the state of Sao Paulo, where flooding and landslides occurred, the official Agencia Brasil news agency reported.

In the city of Sao Paulo, Latin America's biggest metropolis, the storm turned the daytime sky so dark that it appeared to be night, with occasional bolts of lightning and the persistent rumbling of thunder.

Heavy rain submerged 28 spots around the city and brought traffic on normally congested roads to a standstill.

Many flights were delayed at Sao Paulo's main domestic airport and pilots were being forced to rely on instruments because of zero visibility.

In Paraguay, hail stones peppered roofs and damaged some 700 rural properties.

"Damage was registered in the areas of Neembucu, San Pedro, Paraguari, Cordillera, Canindeyu and Caaguazu. Many crops were damaged," the risk manager for the country's emergency service, Aldo Saldivar, told AFP.

The change in weather saw temperatures in the capital Asuncion suddenly plunge from 35 degrees Celsius to 12 degrees (95 degrees Fahrenheit to 54 degrees).