The Zika virus outbreak across the Americas constitutes a health emergency of international concern, World Health Organization chief Margaret Chan said Monday in Geneva.
A WHO board of medical experts sees a connection between the virus and a jump in the number of cases of babies born with undersize skulls, or microcephaly, Chan said.
The experts met earlier in the day to discuss the rapid spread of the Zika virus and whether the outbreak constitutes an emergency of global significance.
The Zika virus is thought to cause birth malformations through mother-to-child transmission, with some 4,180 babies born with unusually small heads amid the Brazilian outbreak. There has also been a spike in reported cases of Guillain-Barre muscle weakness syndrome.
WHO officials said there may have been 1.5 million Zika cases in Brazil, where the outbreak started last year, and that the number of cases in the Americas could grow to 4 million within 12 months.
The WHO last declared an international health emergency in August 2014 with West Africa's Ebola outbreak, allowing the organization to issue rules to combat the epidemic.