WASHINGTON (Reuters) - Thirty-three members of the U.S. military, including a pregnant woman, are believed to have contracted the mosquito-borne Zika virus overseas, the Pentagon said on Wednesday.

Air Force Major Ben Sakrisson, a Pentagon spokesman, said the U.S. service members are believed to have been infected outside the continental United States, but cautioned that it is hard to tell where exactly they may have contracted Zika.

Florida authorities last week reported the first signs of local transmission of the virus in the continental United States in a Miami neighborhood.

The Zika outbreak was first detected last year in Brazil. U.S. health officials have concluded that Zika infections in pregnant women can cause microcephaly, a birth defect marked by small head size that can lead to severe developmental problems in babies.

(Reporting by Susan Heavey and Yeganeh Torbati; Editing by Tim Ahmann and Will Dunham)