The Zika virus - 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.

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Director-general Margaret Chan told members of WHO's executive board in Geneva on Thursday 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.

Argentina confirmed its first case of the virus on Thursday.

Last year the United Nations' health agency was criticised for reacting too slowly to West Africa's Ebola epidemic, which 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 per cent of people infected have no symptoms.

Chan said the WHO will convene an emergency committee meeting on Monday to help determine its response level.

"The level of alarm is extremely high," Chan told WHO executive board members at a meeting in Geneva.

Brazil's Health Ministry said in November that Zika is 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 head of WHO's Americas regional office told Reuters on Thursday.

Brazil has reported 3893 suspected cases of microcephaly, the WHO said last week, more than 30 times more than in any year since 2010 and equivalent to 1-2 per cent of all newborns in the state of Pernambuco, one of the worst-hit areas.

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.

The WHO's 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.

Health and law expert Lawrence Gostin of Georgetown University, who had urged the WHO to act, welcomed Chan's decision to convene an expert meeting, saying in a statement that it was "a critical first step in recognising the seriousness of an emerging epidemic."