The World Health Organization (WHO) says that disease linked to the Zika virus in Latin America which poses a global public health emergency requires a united response.
Experts are worried that the virus is spreading far and fast, with devastating consequences.
However the WHO alert puts Zika in the same category of concern as Ebola.
The U.N. agency took the rare step despite a lack of definitive evidence proving the mosquito-borne virus is causing a surge in babies born with brain defects and abnormally small heads in Brazil and following a 2013-14 outbreak in French Polynesia.
There have been around 4,000 reported cases of microcephaly in Brazil alone since October.
Meanwhile Brazil says that there is no chance that the Rio Olympics will be cancelled because of a Zika virus outbreak.
The authorities said there was no risk to athletes and spectators – except pregnant women – at the August event.