They said there had been 63 confirmed cases of the mosquito-borne illness so far this year, up from seven in the whole of 2016.
Most of the cases have been in rural areas of Minas Gerais state, a Ministry of Health statement said.
The government has sent two million doses of yellow fever vaccines to the state.
The governor of Minas Gerais has declared a 180-day state of emergency.
Source: WHO
Of the 63 confirmed cases in Brazil, 35 have proved fatal, Brazilian Health Ministry figures show.
That is the highest number of deaths since at least 2008, the year to which Ministry of Health records date back.
There have also been three confirmed cases in Sao Paulo, Brazil's most populous state, and one each in Espiritu Santo and Bahia, which both neighbour Minas.
It is not clear what has caused the rise in cases.

A statement from Championship rugby side Pirates says they hope the ground will open in September 2019.
In December, club director Colin Groves estimated construction would begin "in the middle" of this year.
National League South side Truro only agreed to share the proposed 6,000-capacity ground with Pirates in April.
The statement said building will "hopefully" start next spring, with work on the stadium on the outskirts of Truro taking 14 months.
Health officials in Brazil say there has been a sharp rise in the cases of yellow fever in the country.
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Work on the Stadium for Cornwall, to be shared by Cornish Pirates and Truro City, will not start any earlier than spring 2018.