The BBC's David Shukman, reporting from Recife in north-east Brazil, said doctors were "overwhelmed" by cases of microcephaly.
From the section Health
Image caption
Thousands of babies have been born with stunted
brain development in Brazil, which could be linked to Zika virus
The World Health Organization has set up a Zika "emergency team" after the "explosive" spread of the virus.
Zika has been linked to thousands of babies being born with small brains in Brazil.
WHO
director general Dr Margaret Chan said Zika had gone "from a mild
threat to one of alarming proportions" and was having a "heart-breaking"
impact.
The team will meet on Monday to decide whether Zika should be treated as a global emergency.
The
last time an international emergency was declared was for the Ebola
outbreak in West Africa which has killed more than 11,000 people.
Zika link?
Brazil reported the first cases of Zika in South America in May 2015.
Most
cases result in no symptoms and it is hard to test for, but WHO
officials said an estimated 1.5 million people had been infected in the
country.
The virus, which is spread by mosquitoes, has since spread to more than 20 countries in the region. At
the same time there has been a steep rise in levels of microcephaly -
babies born with abnormally small heads - and the rare nervous system
disorder Guillain-Barre syndrome.
The link between the virus and
these disorders has not been confirmed, but Dr Chan said it was
"strongly suspected" and "deeply alarming".
And she warned the
situation could yet deteriorate as "this year's El Nino weather patterns
are expected to increase mosquito populations greatly in many areas". Image copyrightMario TamaImage caption
One hospital in the city of Recife has seen a sharp rise in cases of microcephaly in the past six months
One hospital in the city had gone from dealing with an average of five cases a year to 300 in the past six months.
Emergency team
Earlier, doctors writing in the Journal of the American Medical Association
said Zika had "explosive pandemic potential" and said the WHO's failure
to act swiftly on Ebola probably cost thousands of lives.
In a
statement to the executive board meeting of the WHO, Dr Chan said: "The
level of concern is high, as is the level of uncertainty.
"Questions abound - we need to get some answers quickly.
"For all these reasons, I have decided to convene an Emergency Committee.
"I
am asking the Committee for advice on the appropriate level of
international concern and for recommended measures that should be
undertaken in affected countries and elsewhere."
Dr Carissa
Etienne, the regional-director for the WHO Pan American Health
Organization, said the link between the abnormalities and Zika had not
been confirmed.
But she added: "We cannot tolerate the prospect of
more babies being born with neurological and other malformations and
more people facing the threat of paralysis."
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