Nearly 400 municipalities have been affected by the worst natural calamity ever to hit the state of Rio Grande do Sul, with hundreds of people injured, 128 still missing and 160,000 forced from their homes.

Search and rescue teams were on Wednesday evacuating people trapped in their homes in the state capital of Porto Alegre and other cities and towns.

Authorities, meanwhile, urged people not to return to affected areas due to possible landslide and health hazards.

"Contaminated water can transmit diseases," civil defense spokeswoman Sabrina Ribas said.

The National Confederation of Municipalities said nearly 100,000 homes had been damaged or destroyed by unprecedented rains and floods in the state, with losses estimated at about 4.6 billion reais (more than $900 million.)

Porto Alegre is home to some 1.4 million people and the larger metropolitan area has more than double that number.

The state's Guaiba River, which runs through Porto Alegre, reached historic levels and officials warned Tuesday that five dams were at risk of rupturing.