The World Health Organization has set up a Zika "emergency team" after the "explosive" spread of the virus.
Most people have no symptoms, but WHO expects "three to four million cases of Zika virus disease" in the Americas.
WHO director general Dr Margaret Chan said Zika had gone "from a mild threat to one of alarming proportions" and was having a "heart-breaking" impact.
The team will meet on Monday to decide whether Zika should be treated as a global emergency.
The last time an international emergency was declared was for the Ebola outbreak in West Africa which has killed more than 11,000 people.