Health officials in the southern U.S. state of Florida say three pregnant women have tested positive for the Zika virus.
Florida Health Secretary John Armstrong said the women tested positive after traveling to countries were Zika virus cases were reported.
Florida has a total of 32 cases, with 11 of them reported in the Miami-Dade area. All of the cases have been travel-related.
On Monday, a study to determine whether Zika is causing babies to be born with the birth defect microcephaly began in Brazil.
The Zika virus has been tentatively linked to 4,000 suspected cases in Brazil of microcephaly, a condition that results in abnormally small heads and brains in newborns. There is no treatment for microcephaly.
A 16-member team from the U.S. Centers for Disease Control and Prevention started work in Joao Pessoa, in northeastern Brazil, that is the epicenter of that country's Zika outbreak.
Thomas Frieden, director of the CDC, told VOA that "scientists are working very hard on a vaccine but, it will be at least a year or two before it might be available."
The World Health Organization declared the Zika outbreak an international health emergency February 1.
The CDC said it is investigating 14 new reports of Zika virus infection in the United States, all suspected to be sexually transmitted.
The CDC said all 14 cases involve women whose only known risk factor is sexual contact with a male partner who had recently traveled to an area with local Zika transmissions.