US health officials are investigating 14 reports of the Zika virus that may been transmitted through sex, including to several pregnant women, raising new questions about the role sexual transmission is playing in the growing outbreak.

In two of the suspected cases, the infection has been confirmed in women whose only known risk factor was sexual contact with an infected male partner who had recently travelled to an area with active Zika transmission through mosquito bites, the Centers for Disease Control and Prevention said on Tuesday.

Most experts had believed that sexual transmission of Zika was rare, but the new alert suggests sexual transmission of Zika may be more of a factor than previously thought.

"We think mosquito-borne spread is the most common route of transmission, but we want to make people aware that sexual transmission is also a risk," Jennifer McQuiston, deputy incident manager for CDC's Zika response, said in a telephone interview.

McQuiston said the CDC is investigating the new reports of sexual transmission with the help of state public health departments. In addition to the confirmed cases, the CDC has preliminary test results on four women and the remaining eight cases are in varying degrees of investigation.

All of the newly reported cases of sexual transmission have occurred within the United States. So far, there have been no reports of women transmitting Zika to male sex partners. In a recent study, British researchers reported evidence of the Zika virus in semen of a 68-year-old as long as 62 days after he was first infected.

Zika virus infection generally causes mild symptoms, but it may be linked to thousands of cases of birth defects in Brazil known as microcephaly, which is marked by undersized heads and underdeveloped brains. There is no cure or treatment for Zika infection.

For pregnant women, the CDC recommends that if a male partner has traveled to an area of active Zika transmission that couples use a condom correctly and consistently for the duration of the pregnancy, or to abstain from sex entirely.

"These recommendations might seem extreme to people, but the truth of the matter is we don't yet have good scientific data to say how long the virus may persist in semen," McQuiston said.

She said several studies are planned that will look into this, but until that information is available, it is safer for women to protect themselves during pregnancy.

McQuiston said while these recommendations would also protect women who are not pregnant, the reason for CDC's concern is the "increasing evidence" linking Zika infections to birth defects.

Brazil on Tuesday raised the number of microcephaly cases linked to Zika. It has confirmed 583 cases of microcephaly, up from 508 a week earlier. Suspected cases rose to 4,107 from 3,935.

The first known case of Zika virus transmission in the United States was reported in Texas in early February by local health officials, who said it likely was contracted through sex and not a mosquito bite.

Zika has caused outbreaks in at least 29 countries in the Americas. The CDC added Trinidad and Tobago and Marshall Islands to its travel advisory on Tuesday.

The agency in early February revised its guidelines for pregnant women to include a recommendation that even those without symptoms of the Zika virus should be tested after returning from affected areas. - Reuters