Climate change made the recent flooding that devastated southern Brazil twice as likely, a team of international scientists said on Monday, adding that the heavy rains were also intensified by the natural El Nino phenomenon.

More than 170 people were killed and nearly 580,000 displaced after storms and floods battered Brazil's southernmost state of Rio Grande do Sul last month, with local authorities describing it as the worst disaster in the region's history.

Even in the current climate, experts from the World Weather Attribution group said, the heavy rainfall that submerged entire towns and destroyed critical infrastructure was an "extremely rare" event expected to occur only once every 100 to 250 years.

But it would have been even rarer without the effects of burning fossil fuel, the group said.

By combining weather observations with results from climate models, the scientists estimated that climate change had made the event in southern Brazil twice as likely and around 6% to 9% more intense.

"The climate in Brazil has already changed," said Lincoln Alves, a researcher at Brazil's space research center INPE. "This attribution study confirms that human activities have contributed to more intense and frequent extreme events, highlighting the country's vulnerability to climate change."

The El Nino phenomenon, which contributes to higher temperatures in many parts of the world and boosts rainfall and flood risk in parts of the Americas, also played a part in the recent disaster, the scientists noted.

The study estimated El Nino increased the probability of the event by a factor of 2 to 5, while making the rainfall 3% to 10% more intense.

Failure of critical infrastructure, deforestation and the rapid urbanization of cities such as Rio Grande do Sul's capital Porto Alegre, home to 1.3 million people, helped to amplify the effects of the disaster, the scientists added.

Regina Rodrigues, a researcher at the Federal University of Santa Catarina, said well-maintained flood protection infrastructure and appropriated urban planning are necessary to minimize the impact of "such extreme events".