The World Health Organisation's declaration of the Zika virus as a global emergency was recommended by a committee of independent experts to the United Nations agency.
The designation follows criticism that the WHO's response so far has been 'hesitant'.
It's hoped the move will help fast-track international action and research priorities.
The WHO committee convened to assess the outbreak of the virus after noting a link between Zika's arrival in Brazil last year and a surge in the number of babies born with abnormally small heads.
Director-general of WHO, Margaret Chan, says the spread of the Zika virus is an extraordinary event.
"Members of the committee agree that this situation meets the conditions for a public health emergency of international concern. I am now declaring that the recent clusters of microcephaly and other neurological abnormalities reported in Latin America, following a similar cluster in French Polynesia in 2014, constitutes a public health emergency of international concern."
Ms Chan says an international coordinated response is needed to tackle the disease, although restrictions on travel or trade aren't necessary.
However Ms Chan cautions that there is no definitive proof yet that the virus is responsible for the birth defects.
"In assessing the level of threat, the 18 experts and advisors looked in particular at the strong association in time and in space between infection with the Zika virus and a rise in detected cases of congenital malformations and neurological complications. The experts agree that a causal relationship between the Zika infection during pregnancy and microcephaly is strongly suspected, though not yet scientifically proven."
There have been around 4,000 reported cases of microcephaly - babies born with small brains - in Brazil alone since October.
Brazilian President Dilma Rousseff has authorised health officials to enter private properties by force if necessary in an effort to control the spread of Zika.
Officials are looking for breeding grounds for mosquitoes that can carry the virus.
Brazilian Health Minister Marcelo Castro says the epidemic is worse than believed because in 80 per cent of the cases the infected people have no symptoms.
But he's agreed there isn't enough evidence yet to prove a causal link.
"There is no case like this registered in the literature. What is more, the World Health Organisation has not yet recognised conclusively that it is the Zika virus which causes microcephaly."
Experts believe as the virus spreads from Brazil, other countries in the Americas are also likely to see cases of babies with Zika-linked birth defects.
The Pan American Health Organisation says that Zika has now spread into 24 nations and territories in the Americas.
The disease has no vaccine and no known cure.
Protective measures suggested by health experts include control of mosquito populations and the prevention of mosquito bites.
