Senior U.S. health officials said on Thursday the country's researchers are working on two potential vaccines for the Zika virus, but warned that it will likely take years before they are ready for use.

Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, told a news conference that both candidates were based on earlier vaccines related to the viruses of West Nile and dengue, with one ready for clinical trial later this year.

"While these approaches are promising, it is important to understand that we will not have a widely available safe and effective Zika vaccine this year, and probably not even in the next few years," he said.

Anne Schuchat, principal deputy director of the U.S. Centers for Disease Control and Prevention, said her agency has identified 31 travel-related cases of Zika virus in the United States.

All of the cases involved people returning to the United States from countries where the Zika virus is spreading, she said.

Meanwhile, the U.S. Food and Drug Administration said it's working to rapidly implement appropriate donor deferral measures for travelers who have visited affected regions in order to protect the blood supply.

Zika, transmitted primarily through the bite of an infected Aedes species mosquito, is believed to cause birth defects such as microcephaly, or small heads.

Currently, there is no vaccine to prevent or medicine to treat the disease.

About one in five people infected with Zika virus will develop symptoms, which include fever, rash, joint pain, and pink eye.

The illness is usually mild with symptoms lasting from several days to a week. Severe disease requiring hospitalization is uncommon and the case fatality rate is low.