The death toll from devastating floods and landslides in Brazil rose Monday to 640, as the military stepped up efforts to reach isolated communities near Rio.

The disaster, the worst of its type in Brazil's history, was now mobilising more than 1,500 emergency personnel, including from the army, air force, and police and fire services.

Fears of disease spreading have added urgency to the search for decomposing bodies, and officials have told the local population to not use run-off water for drinking.

Rio de Janeiro state on Monday began seven days of mourning for the victims, adding to a three-day national mourning period declared by President Dilma Rousseff.

The toll looked certain to rise further as roads were cleared to finally allow bulldozers to reach mud-slimed debris in remote hamlets six days after sliding earth swallowed them up.

Some 120 people are missing, presumed dead, according to the Folha de Sao Paulo newspaper, citing state and local officials.

Military and police helicopters were being used to access cut-off areas, after days of rain that had kept them grounded because of limited visibility.

Mayors from the hardest-hit towns of Nova Friburgo, Teresopolis and Petropolis were to meet to discuss how their region, heavily dependent on tourism, can survive, the GloboNews channel reported.

The mudslides that struck the Serrana mountain region just north of Rio de Janeiro on Wednesday last week were caused by rains dumping the equivalent of a month's precipitation in just a few hours.

Destruction was exacerbated by houses illegally built on deforested hillsides -- a situation Rousseff and state officials have blamed on decades of weak municipal oversight in the area.

The disaster was the first big challenge in Rousseff's mandate, who took over from Luiz Inacio Lula da Silva on 1 January.

After visiting the zone last week, Rousseff had her government send $60 million in immediate emergency aid with another $390 million to follow, as well as several tonnes of medical supplies and 700 armed services personnel to help with rescue and recovery operations.

But she also took the weekend off, going to her private residence in the southern city of Porto Alegre to relax.

Observers said that, though she lacked Lula's personable charisma, Rousseff appeared to be passing the test.

"The president was active," political analyst Rodolfo Texeira told AFP, contrasting her performance with that of US president George W Bush's notoriously passive response to Hurricane Katrina, which flooded New Orleans in 2005.

"She sent field hospitals, promised aid, and brought to bear the presence of the state," he said.

Ordinary Brazilians were also rallying to help, sending donations of clothes and food to the disaster zone.

The Estadao news agency reported, however, that many of the packets were being stored outdoors in Teresopolis, where they were becoming sodden.

Refrigerated trucks were parked in front of a makeshift morgue inside a Teresopolis church to take bodies as decomposition and disease became concerns.

A municipal official, Solange Sirico, told Brazilian television there was a risk of epidemics breaking out as bodies decomposing in the tropical heat mingled with water runoff.

"Also, in all the mountain region, there is a danger of snakes, scorpions and spiders," she added.