A Civil Defense statement raised the death toll from 12 to 14 and said nearly 50,000 people were forced to leave their homes, up from the 46,000 reported earlier.
It said 47 cities in Espirito Santo, which borders Rio de Janeiro state, were affected by the flooding, including many left without communications, drinkable water and power.
Rousseff said two helicopters and army trucks would be sent to the area to deliver food and medicine to those affected.
Espirito Santo Governor Renato Casagrande, who Saturday declared a state of alert in the area, said the rains were the worst in the past 90 years.
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Some 13 centimeters (five inches) of rain fell on Sunday night alone in the state capital Vitoria and civil defense officials today warned of further flooding as rivers burst their banks.
The local church and several schools in Itaguacu were turned into makeshift refuge centers.
The rain and landslips have left many roads closed and several bridges have collapsed, with authorities saying two thirds of local districts have been affected.
Rousseff also pledged federal help to Minas Gerais.
Meanwhile in early December, a storm left 16 people dead and flattened more than 200 houses in the city of Lajedinho in the northeastern state of Bahia.