Researchers may need up to six months to find out how Zika fever is connected to malformations in babies, a senior World Health Organization (WHO) official said Friday in Geneva.

The rapid Zika virus outbreak in the Americas has coincided with rising numbers of babies born with abnormally small heads in Brazil, causing WHO to declare a global health emergency in early February.

While scientists have found indications of Zika infections in brains of babies with this malformation, known as microcephaly, as well as in placenta tissue, they have yet to find out exactly what role the virus plays.

The WHO's emergency operations chief Bruce Aylward said babies from mothers who are infected at the start of their pregnancies are at a higher risk, and that scientists will therefore need four to six months to collect further data after more babies are born.

Scientists also have yet to find out why Zika outbreaks have been followed by surges in microcephaly in Brazil and French Polynesia, but not in other countries where the virus has spread.

Zika has also been linked to cases of Guillain-Barre syndrome, a neurological disorder that causes muscle weakness. It might take only weeks to establish this link, Aylward said.

The virus is transmitted by mosquitoes, but there have been a small number of cases of sexual transmission through semen.

Men and women travelling to outbreak regions should practice safer sex for one month after returning home, the WHO advised this week. People living in such areas should always take such precautionary steps, the UN health agency said.

Zika is currently transmitted in 36 countries, most of them in the Americas. The virus usually causes only mild symptoms including fever.

"There has been a steep increase of suspected Zika cases", Aylward said.

Brazilian authorities estimate up to 1.5 million infections, but they have stopped counting due to the magnitude of the outbreak.

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