A Texas county has reported a sexually transmitted case of Zika virus, raising fresh fears about the mosquito-borne infection which has been linked to birth defects.
Dallas County Health and Human Services (DCHHS) said the Dallas patient acquired the virus from sexual contact with an ill individual who had returned from a country where Zika is present.
Health officials said last week that 31 people in the US have been diagnosed with the Zika infection in the past year.
But those infections are thought to have originated through travel to Latin America.
International health officials have been investigating the possibility that the virus can also spread through sex.
Zachary Thompson, DCHHS director, said: "Now that we know Zika virus can be transmitted through sex, this increases our awareness campaign in educating the public about protecting themselves and others.
"Next to abstinence, condoms are the best prevention method against any sexually-transmitted infections."
The Centers for Disease Control and Prevention, a US government agency, has confirmed the Dallas case.
The virus has previously been found in a man's semen in Tahiti.
There was also a report that a Colorado researcher caught Zika overseas and transmitted it to his wife back home in 2008.
The World Health Organization (WHO) warned last week the virus is "spreading explosively" in the Americas.
Zika has been linked to a steep increase in the number of babies born with microcephaly, which causes abnormally small heads and brains.
Cases have so far been reported in 35 countries and territories, with Brazil among the worst affected.
Health officials have advised pregnant women to avoid travel to affected areas.
In several of the affected areas, including El Salvador and Colombia, women have been advised not to get pregnant altogether.
With the exception of the risks for unborn babies, Zika is not considered dangerous.
But there are no known vaccines, specific treatments or rapid diagnostic tests for the virus, something the WHO has highlighted as a particular concern.
Common symptoms can last up to a week and include fever, rashes, joint pain and conjunctivitis.
Brazil is due to host the Olympics this summer and pregnant women have been urged not to attend due to the risk of foetal brain damage.