The zika virus which is linked to severe birth defects in thousands of babies in Brazil is spreading "explosively" and could affect as many as four million people in the Americas, the World Health Organization (WHO) says.
Director-general Margaret Chan said that the spread of the mosquito-borne disease had gone from a mild threat to one of alarming proportions.
"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, promising that the WHO would act fast.
Last year the United Nations' health agency was criticised for reacting too slowly to West Africa's Ebola epidemic, that killed more than 10,000 people, and 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," she said.
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.
Ms Chan said the WHO will convene an emergency committee meeting on 1 February to help determine its response level.
"The level of alarm is extremely high," Ms Chan told the WHO executive board members at a meeting in Geneva.
Brazil's Health Ministry said in November that zika was linked to a foetal deformation known as microcephaly, in which infants are born with abnormally small heads and brains.
As the virus spreads from Brazil, other countries in the Americas are likely to see cases of babies with zika-linked birth defects, the WHO said.
Brazil has reported 3893 suspected cases of microcephaly, the WHO said last week, more than 30 times more than in any year since 2010.
Brazilian president Dilma Rousseff said on Wednesday that the country must wage war against the mosquito that spreads the virus, focusing on eliminating the insect's breeding grounds.
Ms Chan said that while a direct causal relationship between zika virus infection and birth malformations has not yet been established, it is "strongly suspected".
"The possible links, only recently suspected, have rapidly changed the risk profile of zika from a mild threat to one of alarming proportions," she said.
The United States National Institutes of Health's Anthony Fauci, who helped brief President Barack Obama on zika this week, said that federal health agencies were watching zika but do not anticipate a major outbreak there.
"We will see little mini-outbreaks like in Florida or in Texas that can be well-controlled with mosquito vector control. Hopefully, we will not see anything worse than that," he told CBS News.
Asked about the risks for those travelling to Brazil for the summer Olympics, Mr Fauci said aggressively controlling mosquitoes there "is probably the best way".