Nova Friburgo, Brazil (CNN) -- The death toll from devastating flooding in Brazil continued to rise Sunday, surpassing 600, the government said.
At least 631 deaths were reported in a mountainous region of Rio de Janeiro state, northeast of the city of Rio.
Other states in the South American country have also seen heavy rainfall.
Last week, authorities in neighboring Sao Paulo state said 24 people had been killed by flooding. Forecasters there said late Sunday that an approaching cold front could bring more flooding and landslides.
Most of the deaths in Rio de Janeiro state were reported in the cities of Nova Friburgo and Teresopolis, with 287 and 269 fatalities, respectively. The state's health and civil defense department reported 56 fatalities in the town of Petropolis and 19 in Sumidouro.
Officials in that office also warned residents of the risk of waterborne diseases. Several thousand vaccines against tetanus and diphtheria have been distributed, they said.
Rescuers have not been able to reach some hard-hit areas, and many more people are feared dead. The rain is predicted to continue for several days in areas already submerged in water or slathered with mud.
Members of the army entered parts of Teresopolis and were able to rescue 110 families.
Thousands of families are still living on mountain slopes or on riverbanks and face extreme risk of being washed away. One resident described the disaster as a tsunami that fell from the sky.
In a statement, Rio Gov. Sergio Cabral said he had a panic attack when he was traveling to Nova Friburgo and saw a devastated mountainside.
Outside a makeshift morgue in Teresopolis, a crowd of people waited for their turn to identify loved ones.
Marco Antonio Siqueira Costa said the last time he saw his brother, sister-in-law and niece was a few days ago, before mud buried their house.
"I think that last meeting was God's way of granting us a farewell," he said.
Residents in the city donned masks and helped clean streets or deliver first aid. Others combed the city, searching desperately for missing loved ones.
Red Cross volunteer Maria Helena de Jesus was helping with first aid.
"You have to almost have a heart of stone," she said. "It was very difficult."
Teresopolis Mayor Jorge Mario Sedlacek declared his city a natural disaster area.
President Dilma Rousseff flew over flood-affected areas last week and landed in Friburgo, the agency said. The floods are her first test as president.
She trudged through mud to talk to residents in a neighborhood where four of seven firefighters trying to rescue people had been buried under mud. The other three were pulled out alive.
"We are going to take firm action" to help the devastated areas, said Rousseff.
Brazilian authorities have been criticized for a lack of disaster planning and allowing people to build homes in areas known to become treacherous in the rainy season.
They are under increasing pressure to show a strong response. Brazil is scheduled to host the World Cup in 2014 and the 2016 Olympics.
Journalist Fabiana Frayssinet contributed to this report.