Temperatures soared above 40 C (104 Fahrenheit) across large parts of Brazil, Paraguay, Bolivia and Argentina in late winter and lingering into the Southern Hemisphere's spring, with climate change making the event 100 times more likely, said the study by the scientific group World Weather Attribution.

At least four heat-related deaths were reported in Sao Paulo, South America's largest city, but the true death toll is likely to take months to determine by analyzing death certificates, the study said.

"Heat kills, particularly in spring, before people are acclimatized to it. Temperatures above 40 C in early spring are incredibly extreme," said Julie Arrighi, a co-author of the study and director at the nonprofit Red Cross Red Crescent Climate Centre.

A dozen researchers from universities and meteorological agencies around the world produced the study.

This year is on track to be the world's hottest ever recorded, the European Union's Copernicus Climate Change Service said last week.

Summer heat waves recorded in the Northern Hemisphere - including the United States, Europe and China - will be major contributors to that record.

But it's more striking that South America hit such extreme temperatures in the winter, said Gareth Redmond-King, a climate expert at the Energy and Climate Intelligence Unit in London. Redmond-King was not involved in the study.

The onset of the climate phenomenon El Nino this year also helped push temperatures higher, but was a minor factor compared to climate change, the study said.

The study warned that if global warming reaches 2 C hotter than the pre-industrial average, similar heat waves in the region are predicted to happen every five or six years.

The United Nations warned last month that the countries were not doing enough to tackle climate change and current national climate targets have the world on track to warm by 2.5 C.