The World Health Organization (WHO) said on Thursday that the Zika virus is "spreading explosively" and may infect between three to four million people over 12 months throughout the Americas, including one and a half million people in Brazil.
The mosquito borne virus is linked to the birth of thousands of babies with microcephaly in Brazil. The babies are born with unusually small heads and underdeveloped brains.
"The level of concern is high, as is the level of uncertainty... We need to get some answers quickly," WHO’s Director-General Margaret Chan told in a meeting of WHO’s executive body.
"Last year, the virus was detected in the Americas, where it is now spreading explosively. As of today, cases have been reported in 23 countries and territories in the region," she said.
The UN has previously been criticised for reacting too slowly in the Ebola outbreak in West Africa, which killed over 11,000 people.
"We are not going to wait for the science to tell us there is a link [between the virus and birth defects]. We need to take actions now," Chan said.
It could take between six to nine months to prove a link.
Chan said she will convene an emergency committee which will meet in Geneva on Monday.
They will decide in the meeting whether to declare an international state of emergency, which would bring more money and action against the disease in question.
In May 2015, Brazil reported its first case of the Zika virus disease. Since then, the disease has spread within Brazil and to other countries in the region.
On Friday, Germany confirmed five Zika cases between October 2015 and January 2016.
There is no vaccine or treatment for Zika. It was first discovered in Africa in 1947.
Although it causes a mild illness in most people, there is mounting evidence from Brazil that infection in pregnant women is linked to microcephaly.
Brazil has reported around 4,000 suspected cases of microcephaly, vastly more than in an average year
The outbreak has mostly affected the poor and underdeveloped northeast, but the prosperous southeast, where Rio de Janeiro - the host of 2016 Olympics - is located, comes in second.
International Olympic Committee President Thomas Bach said the fact the Olympics will be held in August, during Brazil's winter, could limit Zika's impact on the games.