By Albert Otti

Geneva (dpa) - The fast spread of the Zika virus in the Americas has prompted the World Health Organization to discuss whether the outbreak constitutes a global health emergency, WHO chief Margaret Chan said Thursday in Geneva.

"The level of alarm is extremely high," Chan told WHO member state representatives meeting in Geneva, adding that the virus was being spread "explosively" through mosquitoes in the region.

A panel of WHO-appointed experts is set to meet Monday and to advise Chan whether the situation should be classified as a so-called public health emergency of international concern, as was the case with Ebola.

There are estimates that there may have been 1.5 million Zika cases in this year's Olympic host country Brazil, where the outbreak started last year, senior WHO officials said.

The number of cases in the Americas might grow to 3 million or 4 million within 12 months, they said.

Some 4,000 babies had been born with unusually small heads amid the outbreak, Brazilian Health Ministry official Claudio Maierovitch said via video link to Geneva.

In addition, there has been a spike in cases of the Guillain-Barre muscle weakness syndrome.

"A causal relationship between Zika virus infection and birth malformations and neurological syndromes has not yet been established, but is strongly suspected," Chan said.

Chan emphasized that more research was needed to establish what connections, if any, there are between Zika and these health problems.

In most patients, the Zika virus causes only mild symptoms including fever and rashes.

"It is not Ebola," said Marcos Espinal, WHO's leading communicable disease official in the Americas.

At the same time, Chan said WHO was not only concerned about birth defects and neurological syndromes, but also about the risk of a further international spread, because most people lack immunity, and because there is currently no vaccine or rapid diagnostic method.

"Moreover, conditions associated with this year's El Nino weather pattern are expected to increase mosquito populations greatly in many areas," Chan said, referring to the warming of ocean temperatures that occurs every few years.

The disease has so far reached 24 countries and territories in the Americas.

A man tested positive for the virus in a Danish hospital this week, but local health authorities said there was no risk of an epidemic since the Aedes mosquito is not found in the country.

WHO has recommended pregnant women avoid mosquito bites and consult doctors before planning a trip to countries where the virus is circulating.

WHO official Espinal said the main tasks now were to strengthen health systems in affected countries, step up mosquito control and speed up research on the disease.

"We don't even know yet if this virus crosses the placenta" from mother to child, he said.

The outbreak has hit Brazil at a time when it is already struggling with a recession and is preparing to host the Olympic Games in Rio de Janeiro in August.

Bruce Aylward, WHO's health emergency chief, all but dismissed the notion that his organization would issue a travel warning for Rio, given the current locations of the outbreak hot spots.

"I think that would be very, very unlikely," he told reporters.