The possibility of infection via body fluids could complicate efforts to contain the outbreak.

In fact, the president of the Brazilian federal biomedical research institution that made the announcement urged pregnant women not to kiss strangers during the country's free-wheeling Carnival celebrations.

The US Centers for Disease Control and Prevention recommended more stringent measures for monitoring pregnant women for Zika and for preventing sexual transmission of the virus.

"I wish we knew more about Zika today," CDC Director Tom Frieden told reporters.

The CDC said men with a pregnant partner who live in or have travelled to an area of active Zika transmission should use condoms during sex with their partner or abstain from sex for the duration of the pregnancy.

"The science is not clear on how long the risk should be avoided," the CDC said.

At the centre of the concern over Zika, until recently viewed as a mild illness, is the possibility that infection with Zika during pregnancy may cause microcephaly.

CDC chief Frieden said the suspected link appears "stronger and stronger" as researchers study whether there is a causal connection.

He added that his agency is working with researchers in Brazil to study a potential link between Zika and a wider array of developmental disorders in babies.

Much remains unknown about Zika, including whether the virus actually causes microcephaly. Brazil is investigating the potential link between Zika infections and more than 4,000 suspected cases of microcephaly.

Researchers have identified evidence of Zika infection in 17 of these cases, either in the baby or in the mother, but have not confirmed that Zika can cause microcephaly.

Frieden and other US health officials are due to testify before Congress next Wednesday on the Zika threat.

Brazilians have been rushing to buy repellent, creating a shortage of some brands on pharmacy shelves and boosting sales for the industry - a trend some producers are preparing for elsewhere as the outbreak spreads.