WHO Chief Margaret Chan called for an emergency meeting on February 1 on the outbreak of the virus, which has been blamed for the birth defect microcephaly, a condition that causes babies to be born with an abnormally small head.

"The level of alarm is extremely high," Chan said, adding that the meeting of WHO's Emergency Committee on Monday will seek to determine if the outbreak qualifies as an international public health emergency.

The virus "is now spreading explosively," in the Americas, where 23 countries and territories have reported cases, the WHO Chief said.

Marcos Espinal, the head of communicable diseases and health analysis at the WHO's Americas office, said the region should expect "three to four million cases" of Zika, without proving a time frame for the outbreak to ramp up to that level.

WHO has previously said that it expects Zika to spread to every country in the Americas except Canada and Chile.

Brazil has been the country hardest hit so far, and concerns are growing about this summer's Olympics, which is likely to attract hundreds of thousands of visitors to host city Rio de Janeiro in August.

Microcephaly can cause brain damage or death in babies.

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