The World Health Organization has set up a Zika "emergency team" after the "explosive" spread of the virus.
Zika virus has been linked to thousands of babies being born with small brains in Brazil.
WHO director general Margaret Chan said Zika had gone "from a mild threat to one of alarming proportions" and was "heart-breaking".
The team will meet on Monday and decide whether Zika should be treated as a global emergency like Ebola.
Brazil reported the first cases of Zika in South America in May 2015. WHO said estimates suggested 1.5 million people had been infected in the country.
The virus has since spread to more than 20 countries in the region.
Dr Chan said: "The level of concern is high, as is the level of uncertainty, questions abound, we need to get some answers quickly.
"For all these reasons, I have decided to convene an Emergency Committee.
"I am asking the Committee for advice on the appropriate level of international concern and for recommended measures that should be undertaken in affected countries and elsewhere."
Zika virus has been linked to thousands of babies being born with small brains in Brazil.
WHO director general Margaret Chan said Zika had gone "from a mild threat to one of alarming proportions" and was "heart-breaking".
The team will meet on Monday and decide whether Zika should be treated as a global emergency like Ebola.
Brazil reported the first cases of Zika in South America in May 2015. WHO said estimates suggested 1.5 million people had been infected in the country.
The virus has since spread to more than 20 countries in the region.
Dr Chan said: "The level of concern is high, as is the level of uncertainty, questions abound, we need to get some answers quickly.
"For all these reasons, I have decided to convene an Emergency Committee.
"I am asking the Committee for advice on the appropriate level of international concern and for recommended measures that should be undertaken in affected countries and elsewhere."
Also, brief summary of zika and the latest rapid outbreak
http://www.bbc.com/news/health-35370848
It seems Africa and Asia have long lived with the virus and immunity is stronger there, early negative effects in those continents would easily have gone unnoticed, recently the virus spread island hopped across the pacific and now has found itself a large, susceptible population in the Americas in a modern world where adverse effects are better tracked.
However there is concern of possible confirmation bias
The release of new figures apparently finding fewer cases of microcephaly in Brazil than first feared is adding force to calls for more research into the link between the rare birth defect and the spreading Zika virus.
The WHO is under pressure after the ebola crises, and a strong response is expected.
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