The mosquito-born Zika virus may infect up to four million people, the World Health Organization said, as the agency convened to decide if the outbreak should be declared an international health emergency.
Dr Margaret Chan, WHO director-general, said in a statement on Thursday that the level of alarm was "extremely high".
"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," Dr Chan said.
"Arrival of the virus in some places has been associated with a steep increase in the birth of babies with abnormally small heads and in cases of Guillain-Barre syndrome." The syndrome can cause temporary paralysis.
WHO: Zika virus to spread to much of Americas
Meanwhile, Marcos Espinal, an infectious disease expert at the WHO's Americas regional office, said: "We can expect three to four million cases of Zika virus disease." He gave no time frame, the Reuters news agency reported.

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Dr Chan said a causal relationship between Zika virus infection and birth malformations and neurological syndromes was not yet established, but was "strongly suspected".
She said the emergency committee would advise her on Monday in Geneva on the appropriate level of international concern and on recommended measures that should be undertaken in affected countries and elsewhere. Dr Chan will also ask the committee to prioritise areas where research is most urgently needed.
To step up its fight against the mosquito, Brazil has deployed thousands of municipal, state and federal workers, including soldiers, to scour cities for mosquito breeding grounds, fumigate and educate residents on the dangers of still and stagnant water, where the female insects lay their eggs.
On February 13, the government will deploy 220,000 troops in a one-day mobilisation to hand out leaflets and help identify potential trouble spots.
"All of the countries of Latin America and the Caribbean are trying to coordinate so that they take exactly the same measures to diminish not only the breeding grounds of this mosquito which also carries dengue and chikungunya, but also to prevent it from spreading from one country to another," Al Jazeera's Lucia Newman reported from Santiago, the capital of Chile.
The Zika virus was first detected in 1947 in Uganda, and for decades caused only mild diseases across Africa and equatorial Asia. But Chan noted that "the situation today is dramatically different".
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.
Source: Al Jazeera and agencies