As alarm rose over the latest world health scare, US President Barack Obama called for faster research on the quick-moving virus, which has infected travellers from the United States and Europe.

Brazil has been the country hardest hit by the outbreak of the mosquito-borne virus, which is blamed for a sharp rise in infants born with microcephaly, or abnormally small heads.

The outbreak is particularly concerning officials as the country prepares to host the Olympics, which will bring hundreds of thousands of travellers from around the world to Rio de Janeiro in August.

Zika has spread to some 20 countries in Latin America and the World Health Organisation (WHO) expects it to spread to every country in the Americas except Canada and Chile.

Denmark and Switzerland joined a growing number of European countries to report Zika infections among travellers returning from Latin America.

There is currently no specific treatment for Zika and no way to prevent it other than avoiding mosquito bites.

An American Airlines spokesman said the company was offering refunds to pregnant women with flights booked to Brazil and 10 other countries and territories affected by the virus.

In Brazil, cases of microcephaly have surged from 163 a year on average to more than 3,718 suspected cases since the outbreak, according to the health ministry.

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