The US Centers for Disease Control and Prevention is investigating 14 cases of Zika virus that may have been transmitted through sexual contact. Several of the cases involve pregnant women.
The agency reiterated its call for people to use condoms or practice abstinence when living in or traveling to the more than 20 nations and territories in Latin America and the Caribbean affected by the Zika virus.
The virus is suspected to have played a role in a spike of birth defects in Brazil.
"These new reports suggest sexual transmission may be a more likely means of transmission for Zika virus than previously considered."
Experts still consider Zika a primarily mosquito-borne virus, and urge people to avoid bites by applying insect repellent, using bed nets and wearing long sleeves and pants.
The United States reported its first case of sexually transmitted Zika virus in early February, after a man traveled to Venezuela then returned to Texas and infected a partner who had not traveled there.
"In two of the new suspected sexual transmission events, Zika virus infection has been confirmed in women whose only known risk factor was sexual contact with an ill male partner who had recently traveled to an area with local Zika virus transmission," said the CDC.
"Testing for the male partners is still pending."
The CDC said the 14 cases, "like previously reported cases of sexual transmission... involve possible transmission of the virus from men to their sex partners."
Health officials currently have "no evidence that women can transmit Zika virus to their sex partners; however, more research is needed to understand this issue."
There is no vaccine to prevent Zika virus and no medicine to treat it.
In four out of five cases, people who are bitten by an infected mosquito show no symptoms.
Otherwise, symptoms may include a rash, red eyes and a headache.
But in pregnant women, the consequences could be dire.
Scientists are currently trying to figure out if Zika could have caused an increase in birth defects in Brazil, where thousands of babies have been born with unusually small heads since the virus began to spread there.