Brazil's top health official said on Monday that the Zika virus outbreak is proving to be worse than believed because most cases show no symptoms, but improved testing should allow the country to get a better grip on the epidemic.

Health Minister Marcelo Castro told Reuters that Brazil will start mandatory reporting of cases by local governments next week when most states will have labs equipped to test for Zika, the mosquito-borne virus that has quickly spread through Latin America.

The rapid arrival of Zika to Brazil has sparked fear especially among pregnant women after local experts linked the virus to thousands of cases of microcephaly, or abnormally small brains, in newborns.

"Eighty percent of the people infected by Zika do not develop significant symptoms. A large number of people have the virus with no symptoms, so the situation is more serious that we can imagine," Castro said in an interview.

With no Zika vaccine available for the foreseeable future, Brazil's only option is to eradicate the mosquito that has spread the virus, and the government is mobilizing all possible resources and people to destroy its breeding places, he said.

An estimated 1.5 million Brazilians have caught Zika, a virus first detected in Africa in the 1940s and unknown in the Americas until it appeared in May in the poverty-stricken northeastern region of Brazil.

The Pan-American health Organization said the virus has since spread to 24 countries and territories in the hemisphere.

By next week, labs in all but three of Brazil's states will be able to test whether a person has had Zika or not, Castro said.

By next month, the labs will have a test that can detect all three viruses borne by the Aedes aegypti mosquito - dengue, chikungunya and Zika. The test, however, will only be effective during the initial infection period of five days.

Castro said Brazilian researchers are convinced that Zika is the cause of the 3,700 confirmed and suspected cases in Brazil of microcephaly in newborns. He said the virus cannot be transmitted from person to person, only by mosquito.

Brazil will follow the U.S. decision last week to prohibit blood donations from people who have been infected with Zika, he said.

The U.S. Food and Drug Administration, however, has said it is planning to require people who have traveled to an affected country to defer giving blood, but details on how that might work are still being determined.