More than 300mm of rain fell in 24 hours in the Brazilian state of Rio Grande do Sul on Monday and more is expected on Thursday.
The death toll from a cyclone that heavy rains in southern Brazil has risen to 36, authorities said on Wednesday.
Thousand of people were forced from their homes by the storms, which started on Monday (4 September).
Hail and more than 300 millimetres (nearly 12 inches) of rain were dumped on the state of Rio Grande do Sul in less than 24 hours, triggering floods and landslides.
Governor Eduardo Leite said the death toll was the highest the state had ever seen due to a climate event. He told a news conference that 15 more bodies had been found on Tuesday, bringing the total dead to 21.
How bad are the floods in Brazil?
The storm, which was classified as an extratropical cyclone, hit 67 municipalities in all, affecting more than 52,000 people, authorities said.
In Muçum, a small town of 5,000 people, hundreds had to be rescued from their rooftops as the Taquari River flooded more than 85 per cent of the city, according to local news site GZH.
Video footage shows some homes almost completely submerged with only their roofs above the water.
Hundreds of firefighters as well as military police and civil defence personnel were dispatched as part of rescue operations, with helicopters sent to reach areas cut off by flooding.
“There are still people missing. The death toll might climb higher,” Mayor Mateus Trojan told Radio Gaucha.
“The town of Muçum as we knew it no longer exists.”
The neighbouring state of Santa Catarina also recorded one death, according to news site G1.
Is Brazil at further risk of flooding?
With more rain forecast from Thursday, authorities warn more flooding is possible.
It is the latest in a string of deadly weather events to hit Brazil, which experts say are likely being made worse by climate change.
Unchecked urbanisation and irregular housing built on hillsides are also making such disasters deadlier, officials say.
An estimated 9.5 million of Brazil’s 203 million people live in areas at high risk of flooding or landslides.
In June, another cyclone left 13 dead in Rio Grande do Sul and forced thousands of people from their homes.
In February, 65 people died in landslides caused by record flooding in the southeastern resort town of Sao Sebastiao, on the coast of Sao Paulo state.
Watch the video above to see the scale of the flooding in Rio Grande do Sul, Brazil.
Video editor • Hannah Brown