The World Health Organisation (WHO) has declared the rise in cases of brain damage in newborn babies caused by Zika virus a global public health emergency.
WHO Director-General Margaret Chan said a global response was needed to contain the outbreak that is linked to brain damage in thousands of children in Brazil.
Chan said it was "strongly suspected but not yet scientifically proven" that Zika causes microcephaly, a foetal deformation in which infants are born with smaller-than-usual brains.
But she said it was not wise to wait for a scientific confirmation, adding that the first and foremost concern was about microcephaly.
"Can you imagine if we do not do all this work now, and wait until the scientific evidence comes out?" ... Then people will say that, 'Why don't you take action because the mosquito is ubiquitous?'" she told reporters at the WHO headquarters in Geneva.
WHO had been accused of slow response to the Ebola epidemic in West Africa that killed more than 10,000 people.
David Heymann, chairman of the WHO Emergency Committee, said the decision to declare the outbreak a global emergency was a difficult one as Zika alone is not a clinically-serious illness in most people.
WHO officials occluded that global alert will raise attention to the outbreak and prompt governments and health agencies fight the virus on various fronts such as better mosquito control measures and the development of a preventive vaccine.
The global agency had said last week the mosquito-borne disease might spread to four million people in the Americas.
According to the WHO the virus has spread to 25 countries in the central and south Americas. Cases have also been reported in the southeast Asia, Europe and New Zealand.
In Colombia, more than 2,000 pregnant women were infected with the virus, reports said, raising alarm about the disease that has the worst impact on pregnant women.
The virus doesn't spread directly from person to person. Vaccines are not available to counter the disease and there is no specific treatment for it. Meanwhile, scientists in the US, who likened Zika outbreak to the Ebola crisis, said it could be years before a vaccine is publicly available.