U.S. health officials are investigating more than a dozen possible Zika infections that may have been spread through sex.

The 14 cases all involve men who visited areas with Zika outbreaks, and who may have infected their female sex partners, who had not travelled.

Zika virus is mainly spread by mosquito bites, and sexual transmission has been considered rare. There have been two reported cases, including a recent one in Texas, and at least two other reports of the Zika virus found in semen.

Mosquito-borne Zika outbreaks have erupted across most of Latin America and the Caribbean in the past year.

On Tuesday, the Centers for Disease Control and Prevention said the 14 possible U.S. cases include two pregnant women whose infections have been confirmed. Tests are pending for their male partners.

The agency said there's no evidence that women can spread the virus to their sex partners, but more research is needed.

In most people, Zika causes mild or no symptoms — fever, joint pain, rash and red eyes — that last about a week. But in Brazil, health officials are investigating a possible connection between the virus and babies born with brain defects and abnormally small heads.

The link hasn't been confirmed but the possibility has prompted health officials to take cautionary steps to protect fetuses from the virus.

Research is also underway into a possible link between Zika infection and a paralyzing condition in adults called Guillain-Barre syndrome.

The CDC is advising men who have recently been to a Zika outbreak area to use a condom when they have sex with a pregnant woman, or to abstain from sex during the pregnancy. It also has recommended that pregnant women postpone trips to more than 30 destinations with outbreaks.

The CDC on Tuesday expanded its Zikatraveladvisory to two more places — the Marshall Islands, and Trinidad and Tobago.

There is no vaccine for Zika. Researchers are scrambling to develop one, as well as better diagnostic tests.