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.
What is yellow fever?
§ Caused by a virus that is transmitted to humans by mosquitoes
