The Zika virus, linked to severe birth defects in thousands of babies in Brazil, is "spreading explosively" and could infect as many as 4 million people in the Americas, the World Health Organization (WHO) said on Thursday.
Director-General Margaret Chan told members of the U.N. health agency's executive board the spread of the mosquito-borne disease had gone from a mild threat to one of alarming proportions.
The WHO would convene an emergency meeting on Monday to help determine its response, she said.
"The level of alarm is extremely high," Chan told the Geneva gathering.
"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," Chan said, promising quick action from the WHO.
The agency was criticized last year for reacting too slowly to West Africa's Ebola epidemic, which killed more than 10,000 people, and it promised to cut its response time.
"We are not going to wait for the science to tell us there is a link (with birth defects).We need to take actions now," Chan said, referring to the condition called microcephaly in which babies are born with abnormally small heads and brains that have not developed properly.
There is no vaccine or treatment for Zika, which is like dengue and causes mild fever, rash and red eyes.
An estimated 80 percent of people infected have no symptoms.
Much of the effort against the illness focuses on protecting people from mosquitoes and reducing mosquito populations.
Developing a safe and effective vaccine could take a year, WHO Assistant Director Bruce Aylward said, and it would take six to nine months just to confirm whether Zika is the actual cause of the birth defects, or if the two are just associated.
"In the area of vaccines, I do know that there has been some work done by some groups looking at the feasibility of a Zika virus vaccine. Now something like that, as people know, is going to be a 12-month-plus time frame," he said.
