Torrential rain and winds caused by an extratropical cyclone have left at least 21 people dead in southern Brazil, officials said Tuesday, warning more flooding may be coming.

The latest in a string of weather disasters to hit Brazil, it is the deadliest ever in the state of Rio Grande do Sul, Governor Eduardo Leite told a news conference.

"We were deeply saddened to get the news that as the water recedes... 15 more bodies were found in the town of Mucum, bringing the death toll to 21," he said.

Close to 6,000 people were forced from their homes by the storms, which started Monday, dumping hail and more than 300 millimeters (nearly 12 inches) of rain on the state in less than 24 hours and triggering floods and landslides, officials said.

In Mucum, a small town of 5,000 people, hundreds had to be rescued from their rooftops as the Taquari river flooded more than 85 percent of the city, according to local news site GZH.

"There are still people missing. The death toll might climb higher," Mayor Mateus Trojan told Radio Gaucha.

"The town of Mucum as we knew it no longer exists."

President Luiz Inacio Lula da Silva sent a message of solidarity with those affected, saying the federal government is "ready to help."

- Swept away -

The victims included a man killed by an electrical shock in the town of Passo Fundo and a couple whose car was swept away by a river as they tried to cross a bridge in the town of Ibiraiaras.

The storms hit 67 municipalities in all, affecting more than 52,000 people, authorities said.

The neighboring state of Santa Catarina also recorded one death, according to news site G1.

Hundreds of firefighters as well as military police and civil defense personnel were dispatched as part of rescue operations, with helicopters sent to reach areas cut off by flooding.

"There are many isolated families, many people still at risk," said Communications Minister Paulo Pimenta, who is planning to travel to the region with a government delegation Wednesday.

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 urbanization 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.

And 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.