RIO DE JANEIRO—The death toll from severe flooding in southeastern Brazil approached 500 people Thursday, as further rain threatened to complicate relief and recovery efforts after walls of mud and water gushed across entire towns in recent days.

After a helicopter flyover and visit to some of the affected areas in the state of Rio de Janeiro, President Dilma Rousseff, in her second week in office, vowed to cut bureaucracy that could hinder prompt delivery of federal funding and aid for overwhelmed state and local officials.

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