The U.N. health agency convened an emergency meeting of independent experts in Geneva to assess the outbreak after noting a suspicious link between Zika's arrival in Brazil last year and a surge in the number of babies born with abnormally small heads.
"After a review of the evidence, the committee advised that the clusters of microcephaly and other neurological complications constitute an extraordinary event and public health threat to other parts of the world," WHO Director-General Margaret Chan said.
WHO estimates there could be up to 4 million cases of Zika in the Americas in the next year, but no recommendations were made to restrict travel or trade.
"It is important to understand, there are several measures pregnant women can take," Chan said. "If you can delay travel and it does not affect your other family commitments, it is something they can consider.
"If they need to travel, they can get advice from their physician and take personal protective measures, like wearing long sleeves and shirts and pants and use mosquito repellent."
The last such public health emergency was declared for the devastating 2014 Ebola outbreak in West Africa, which killed more than 11,000 people. A similar declaration was made for polio the year before.
Such emergency declarations are meant as an international SOS signal and usually trigger increased money and efforts to stop the outbreak, as well as prompting research into possible treatments and vaccines.
WHO officials say it could be six to nine months before science proves or disproves any connection between Zika and the spike in the number of babies born in Brazil with abnormally small heads.
WHO, which was widely criticized for its slow response to the 2014 Ebola crisis in West Africa, has been eager to show its responsiveness this time. Despite dire warnings that Ebola was out of control in mid-2014, WHO didn't declare an emergency until August, when nearly 1,000 people had died.
Its officials say that up to 4 million cases of Zika could turn up in the Americas within the next year. Zika was first identified in 1947 in a Ugandan forest but until last year, it wasn't believed to cause any serious effects; about 80 percent of infected people never experience symptoms. The virus has also been linked to Guillain-Barre syndrome, which causes muscle weakness and nerve problems.
