Brazilian President Luiz Inacio Lula da Silva on Thursday visited the country's south where floods and mudslides caused by torrential rains have killed 29 people, with the toll expected to rise.
Authorities in Rio Grande do Sul have declared a state of emergency as rescuers continue to search for dozens of people reported missing among the ruins of collapsed homes, bridges and roads.
Storm damage has affected nearly 150 municipalities in the state, also injuring 36 people and displacing more than 10,000.
Governor Eduardo Leite said Rio Grande do Sul was dealing with "the worst disaster in (its) history."
"With the deepest pain in my heart, I know it will be even more," the governor said of the death toll.
Lula, who has blamed the torrent on climate change, arrived in the town of Santa Maria in the morning with a delegation of ministers and held a working meeting with Leite and other officials to coordinate rescue efforts, the government said.
The president promised "there will be no lack of human or material resources" to "minimize the suffering this extreme event... is causing in the state."
The federal government, he added, "will be 100 percent at the disposition" of state officials.
Central authorities has already made available 12 aircraft, 45 vehicles and 12 boats as well as 626 soldiers to help clear roads, distribute food, water and mattresses, and set up shelters, a press statement said.
As the rains continued, forecasts warned the state's main Guaiba River, which has already overflowed its banks in some areas, would reach an extraordinary level of three meters (9.8 feet) by Thursday and four meters the next day.
- 'Completely destroyed' -
Entire communities in Rio Grande do Sul state have been completely cut off as persistent rains have destroyed bridges and blocked roads, and left towns without even telephone or internet services.
Rescuers and soldiers have been scrambling to free families trapped in their homes, many stuck on rooftops to escape rising waters.
"I've never seen anything like this... it's all under water," said Raul Metzel, a 52-year-old machine operator in the municipality of Capela de Santana.
A dam collapsed in the town of Cotipora, raising the level of water in the Taquari river.
"I came here to help people, to get them out of the flooding because it is very dangerous. The current is very strong," said fisherman Guilverto Luiz, who was helping rescue efforts in Sao Sebastiaio do Cai, about 70 km from Porto Alegre, the state capital.
Authorities have urged people to avoid areas along state highways due to a risk of mudslides, and those who live near rivers or on hillsides to evacuate.
Hundreds of thousands of people have been left without access to electricity and drinking water, while classes have been suspended state-wide.
On Wednesday, the state's deputy governor, Gabriel Souza, said damages have been estimated at $20 million.
Mayor Sandra Backes of Sinimbu said the situation in her town was "a nightmare."
"Sinimbu is like a war zone, completely destroyed... All the stores, businesses, supermarkets -- everything is devastated," she said in a video posted on Instagram.
Elsewhere, in Santa Cruz do Sul, lifeguards used boats to transport residents, many of them children, to safety.
The region's rivers had already been swollen from previous storms.
Last September at least 31 people died as a cyclone hit the state.
South America's largest country has suffered a string of recent extreme weather events, which experts say are made more likely by climate change.
The floods came amid a cold front battering the south and southeast, following a wave of extreme heat.