WASHINGTON • The US health authorities said they are investigating 14 new reports of the Zika virus possibly being transmitted by sex, including to pregnant women. If confirmed, the high number would have major implications for controlling the virus, which is usually spread by mosquito bites.

Scientists had believed sexual transmission of Zika to be extremely rare. Only a few cases have ever been documented. But if all the women in the cases the Centres for Disease Control and Prevention (CDC) is examining test positive for the virus - as two women already have, and four others had in preliminary lab tests - officials believe there is no way other than sex that they could have contracted it.

The spectre of so many cases - all in the continental United States - brings fresh complexity to the medical mystery of Zika. The virus is suspected to cause birth defects and a rare condition of temporary paralysis.

"We were surprised that there was this number," Dr Anne Schuchat, the deputy director at the CDC, said in an interview. "If a number of them pan out, that's much more than I was expecting."

In all the cases the CDC is examining, women in the continental US had sex with men who had travelled to countries where the virus is circulating, and developed symptoms associated with the virus within about two weeks of their male partner's symptoms.

Officials at the CDC reported the potential cases in an alert to health care providers on Tuesday.

The US has become a laboratory of sorts to test the sexual transmission of Zika, as scientists race to understand the disease.

Cases of active Zika transmission have been reported in 28 countries and territories in the Americas and Caribbean.

Meanwhile, the head of the World Health Organisation expressed confidence on Tuesday that Brazil can host the Rio Olympics safely despite the Zika threat, while warning of a long battle against the mosquito-borne virus.

We must "make sure that people who come here for the Games, either as a visitor or as a participant, will get the maximum protection they need", Dr Margaret Chan said after meeting President Dilma Rousseff and several ministers in the capital Brasilia.

"And I'm confident that the government can do it."

NEW YORK TIMES, AGENCE FRANCE-PRESSE