Travel warnings to pregnant women have been extended to eight more countries or territories amid concerns over an illness causing severe birth defects.
On Wednesday, Brazil said the number of babies born with suspected microcephaly or abnormally small heads since October had reached nearly 4,000.
The Brazilian authorities believe the increase is caused by an outbreak of the mosquito-borne Zika virus.
Warnings by US health officials now cover parts of Africa and Oceania.
One leading virologist in Brazil told the BBC the country was in an "emergency situation".
Brazil's health ministry says there have been 3,893 suspected cases of microcephaly since October, when the authorities first noticed a surge, up from 3,500 in last week's report.
What is Zika virus?
- It is spread by the Aedes aegypti mosquito, which also carries dengue fever and yellow fever
- It was first discovered in Africa in the 1940s but is now spreading in Latin America
- Scientists say there is growing evidence of a link to microcephaly, that leads to babies being born with small heads
- While Zika virus can lead to fever and a rash, most people show no symptoms, and there is no known cure
- The only way to fight Zika is to clear stagnant water where mosquitoes breed, and to protect against mosquito bites
The link between microcephaly and Zika has not been confirmed but a small number of babies who died had the virus in their brain and no other explanation for the surge in microcephaly has been suggested.
The brain condition can be deadly or cause intellectual disability and developmental delays.
Forty-nine babies with suspected microcephaly have died, Brazil's health ministry says. In five of these cases an infection with Zika virus was found.
Brazil is experiencing the largest known outbreak of Zika, with most cases in the north-east. Others have been detected in the south-east, an area which includes Rio de Janeiro and Sao Paulo.
There has been a sharp rise in the number of cases of Zika in several other Latin American countries.
In Colombia, more than 13,500 cases have been reported, and the country's health minister has advised women there to delay pregnancy.
The US Centers for Disease Control and Prevention issued initial travel warnings to pregnant women last week, adding eight more places to the list on Friday. The warnings now extend to
- Central and South America: Bolivia, Ecuador, Guyana, Brazil, Colombia, El Salvador, French Guiana, Guatemala, Honduras, Mexico, Panama, Paraguay, Suriname, Venezuela
- Caribbean: Barbados, Saint Martin, Haiti, Martinique, Puerto Rico, Guadeloupe
- Oceania: Samoa
- Africa: Cape Verde
"The virus found the perfect conditions in Brazil," Ricardo Lourenco, who studies tropical infectious diseases at Brazil's Oswaldo Cruz Institute, told Reuters news agency. "A very efficient vector that loves human blood, millions of susceptible victims with no antibodies, ideal climate and lots of places to breed."
Davis Ferreira, a virologist with the Federal University of Rio de Janeiro, told the BBC Brazil was facing a crisis similar to the one West Africa faced with Ebola: "We have newborns, thousands of newborns with microcephaly.
"And we don't know what's to come. We're in a emergency situation."
Rio de Janeiro is due to host the Olympics in August. The country is expecting 10,500 international athletes and many more spectators to attend.