The World Health Organization (WHO) has announced that the explosive spread of the Zika virus in the Americas is an "extraordinary event" that merits being declared an international emergency.

The agency convened an emergency meeting of independent experts on Monday to assess the outbreak, after noting a link between Zika's arrival in Brazil last year and a surge in the number of babies born with abnormally small heads.

Margaret Chan, WHO director-general said there is an "urgent need to coordinate international efforts to understand whether the Zika virus is causing birth defects."

The WHO is under pressure to act quickly in the fight against Zika, after admitting it was slow to respond to the recent Ebola outbreak that ravaged parts of West Africa and killed more than 11,300 people.

The WHO declared that the surge in South America was "strongly suspected" of being caused by the Zika virus.

The UN health body said that there was a causal relationship between the mosquito-borne Zika virus and a surge in cases of microcephaly - the devastating condition which also sees babies born with small brains.

WHO estimates there could be up to 4 million cases of Zika in the Americas in the next year.

Emergency declarations are meant as an international SOS signal and usually trigger increased money and efforts to stop the outbreak, as well as prompting research into possible treatments and vaccines.

'It is going to spread'

The declaration came amid news that Panama said it has 50 cases of Zika virus infections and warned that Zika will end up spreading across the Central American nation.

"Let's be clear: it [Zika] is going to enter, it is going to spread," the head of the health ministry's epidemological department, Israel Cedeno, told the television network TVN-2.

The 50 cases confirmed so far in Panama were concentrated in the predominantly indigenous Guna Yala region along its Caribbean coast.

Vice President Isabel De Saint Malo last week had spoken of 38 cases in Guna Yala and said at the time that "there is no big public health risk."

Infections have been reported in 13 countries in the Americas, according to WHO, as well as in Asia, and in Africa, from where it originated.

Panama borders Colombia, which has so far reported more than 20,000 cases of Zika, including 2,100 in pregnant women. Colombia is forecasting it will see more than 650,000 infections.