More than two million people have been affected by the flooding in the state of Rio Grande do Sul, where towns and part of the regional capital have been under water for about two weeks after being battered by heavy rains.
Regional capital Porto Alegre's international airport remains under water, as do agricultural fields and roads, while more than 360,000 students were not in school.
The latest official figures showed that 127 people were missing, while more than 600,000 had been forced to abandon their homes as rivers burst their banks.
The floods are the latest weather extreme to hit Brazil, after record-breaking forest fires, unprecedented heat waves and drought, and experts have attributed the disaster to climate change exacerbated by the El Nino phenomenon.
Rains eased on Monday, but fresh downpours over the weekend in the region had led rivers to swell once again, raising fears of further flooding and damage.
"It is not the moment to return to homes in risk zones," state governor Eduardo Leite told a press conference.
'Endless drama'
Brazil's President Luiz Inacio Lula da Silva put off a state visit to Chile this week to focus on the disaster.
The presidency said the trip was delayed due to the "need to monitor the situation linked to flooding in Rio Grande do Sul and coordinate aid to the affected population and reconstruction efforts."
The Guaiba, an estuary bordering Porto Alegre which overflows when waters hit three meters, reached a historic high of 5.3 meters last week.
Authorities have warned it could still swell to a new record of 5.6 meters after the weekend deluge.
"We are experiencing the aftermath of an endless drama here in Rio Grande do Sul," the state's deputy governor Gabriel Souza told the Globo broadcaster.
In Porto Alegre, a busting state capital of 1.4 million inhabitants, aid operations continued to deliver food, clean water, medicine and clothing to residents.
This is "the largest logistics operation in the history of the state," said Leite.
Among those seriously affected are about 80 Indigenous communities, according to the Indigenous Missionary Council of Brazil.
The government said Monday it had delivered food kits and drinking water for 240 Indigenous families in the Taquari Valley.
The heavy rains in southern Brazil have also led to the flooding of the Uruguay River which flows between Brazil, Argentina, and Uruguay.
Argentine authorities reported some 600 people had to be evacuated in the riverside city of Concordia.
Concordia mayor Francisco Azcue said waters were expected to continue to rise in the coming days.
"Obviously we are going to have more evacuees," he said.
