The World Health Organisation has said it is convening an emergency committee on Monday to decide if the Zika virus outbreak should be declared an international health emergency.
At a special meeting, WHO director-general Dr Margaret Chan said the virus - which has been linked to birth defects and neurological problems - is "spreading explosively".
Dr Chan said although there was no definitive proof that Zika 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".
The WHO last declared an international emergency over the devastating 2014 Ebola outbreak in West Africa, which ended up killing more than 11,000 people.
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 Centres for Disease Control, the virus is now in more than 20 countries, mostly in Central and South America.
Dr Chan 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," Dr Chan said.
There is no specific treatment or vaccine for Zika, which is related to dengue - scientists have struggled for years to develop a dengue vaccine but have failed to create a viable shot so far.
The 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 the WHO is "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.
But convening an emergency committee does not guarantee that a global emergency will be declared - the WHO has held 10 such meetings to assess the Middle Eastern respiratory syndrome coronavirus and no emergency has been announced.
One reason the UN health agency may be examining the Zika virus so quickly is because the WHO was criticised for its slow response to Ebola; nearly 1,000 people had died before the agency declared it an international emergency.
The WHO estimates there could be three to four million cases of Zika in the Americas over the next year.
Sylvain Aldighieri, head of the WHO's epidemic response team in the Americas, said the estimate is based on previous numbers of infections of dengue fever, which is also carried by mosquitoes.
He said the agency expects "huge numbers" of infections because of the widespread presence of the mosquitoes that spread Zika and because there is no immunity among the population.
He said that since most people with Zika do not get sick, there is a "silent circulation" of the disease that may make tracking its spread more difficult.
China and other countries with dengue fever should be on the lookout for Zika virus infections, the WHO said.
Dr Bruce Aylward, who runs the WHO's outbreak response department, said any country that has the Aedes mosquito should be concerned about the possibility of the Zika virus arriving.
The Aedes mosquito spreads diseases including Zika, dengue fever and chikungunya.