U.S. 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, Reuters reports.

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 traveled to an area with active Zika transmission through mosquito bites, the Centers for Disease Control and Prevention said on Tuesday, Feb 23.

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," Reuters quoted Jennifer McQuiston, deputy incident manager for CDC's Zika response, as saying.

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.

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.