Torrential rain and winds caused by a cyclone have left at least 27 people dead in southern Brazil, with more flooding expected.
The governor of Rio Grande do Sul said it was the state's worst-ever weather disaster.
Thousands have been forced from their homes, officials said.
In Mucum, a town of 5,000 people, hundreds had to be rescued from their rooftops as 85% of the town was flooded, local media report.
President Luiz Inácio Lula da Silva said the federal government was ready to help.
"Where there is a problem, the federal government will be there to save people from these problems," he said.
Eduardo Leite, governor of Rio Grande do Sul, told a news conference that 15 more bodies had been found in Mucum on Tuesday, bringing the death toll to 21.
More than 300mm (11 inches) of rain hit the state in less than 24 hours, triggering floods and landslides, officials said.
"There are still people missing," Mucum Mayor Mateus Trojan told Radio Gaucha.
"The death toll might climb higher. The town of Mucum as we knew it no longer exists."
Rescue workers have been using helicopters to reach areas cut off by flooding.
Further north in the town of Nova Bassano, resident Dirce Reginatto said she was "devastated".
"There are many people who lost much more, but here at home I have nothing left," she told Reuters news agency.
Luana Da Luz, from Passo Fundo, was rescued by firefighters.
"We saw it [water] was going to fill everything since dawn," she said. "We were already putting things on top of the table, on top of the wood stove, but it didn't help."
Many factors contribute to flooding, but a warming atmosphere caused by climate change makes extreme rainfall more likely.
The world has already warmed by about 1.2C since the industrial era began and temperatures will keep rising unless governments around the world make steep cuts to emissions.
In February, at least 40 people were killed in flooding and landslides in Brazil's São Paulo state. while last year at least 100 people died as torrential rain triggered landslides and torrents of mud near the city of Recife in the country's north-east.