At a special meeting in Geneva, WHO director-general Margaret Chan said the virus — which has been linked to birth defects and neurological problems — is becoming more of a threat.
Dr Chan said although there was no definitive proof that the zika virus was responsible for a spike in the number of babies being born with abnormally small heads in Brazil, “the level of alarm is extremely high”.
She also noted a possible relationship between zika infection and Guillain-Barre syndrome, which can cause temporary paralysis.
“The possible links, only recently suspected, have rapidly changed the risk profile of zika from a mild threat to one of alarming proportions,” said Dr Chan.
“The increased incidence of microcephaly is particularly alarming, as it places a heart-breaking burden on families and communities.”
The zika virus was first detected in 1947 and for decades only caused mild disease, but Dr Chan noted that “the situation today is dramatically different.”
According to the US Centers for Disease Control, the zika virus is now in more than 20 countries, mostly in Central and South America.
It is spread by the Aedes mosquito, which also spreads dengue and yellow fever.
WHO called the special session in part to convey its concern about an illness that has sown fear among many would-be mothers, who have responded by covering themselves head-to-toe in clothing in largely tropical Brazil or putting on many coats of insect repellent.
Dr Chan cited four main reasons why WHO was “deeply concerned” about zika: The possible link to birth defects and brain syndromes, the prospect of further spread, a lack of immunity among people living in the newly-affected areas, and the absence of vaccines, treatments, or quick diagnostic tests for the virus.
Declaring a global emergency is akin to an international SOS signal and usually brings more money and action to address an outbreak.
The last such emergency was announced for the devastating 2014 ebola outbreak in West Africa, which eventually ending up killing over 11,000 people.
Polio was declared a similar emergency the year before.
Still, convening an emergency committee does not guarantee that a global emergency will be declared — WHO has held 10 such meetings to assess the Middle Eastern respiratory syndrome coronavirus and no emergency has been announced.
Here's what you need to know about the Zika virus https://t.co/xzsZhrcDg1— The New York Times (@nytimes) January 28, 2016