The World Health Organization (WHO) said on Tuesday that it was gearing up to combat the microcephaly outbreak in Latin America linked to the Zika virus, which it feared could spread to Asia and Africa that have "the highest birth rates in the world".

Anthony Costello, a WHO expert, said that the U.N. agency was drawing up "good guidelines" for pregnant women and gathering experts to work on a definition of microcephaly including a standardized measurement of babies' heads.

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Costello, referring to the link in Brazil between the Zika virus and microcephaly, a condition where babies are born with small heads, said: "We believe the association is guilty until proven innocent."

"Mass community engagement" in areas with the mosquito that carries the Zika virus, and rapid development of diagnostic tools were essential, while a vaccine may be years away, he said.