Margaret Chan, WHO director-general, said in a statement on Thursday that the level of alarm was "extremely high".
"Last year, the virus was detected in the Americas, where it is now spreading explosively. As of today, cases have been reported in 23 countries and territories in the region," Chan said.
"Arrival of the virus in some places has been associated with a steep increase in the birth of babies with abnormally small heads and [with] cases of Guillain-Barre syndrome." The syndrome can cause temporary paralysis.
Meanwhile, Marcos Espinal, an infectious disease expert at WHO's Americas' regional office, said: "We can expect three to four million cases of Zika virus disease." He gave no time frame, the Reuters news agency reported.
Chan said a causal relationship between Zika virus infection and birth malformations and neurological syndromes was not yet established, but was "strongly suspected".
She said the emergency committee would advise her on Monday in Geneva on the appropriate level of international concern and on recommended measures that should be undertaken in affected countries and elsewhere. Chan will also ask the committee to prioritise areas where research is most urgently needed.
To step up its fight against the mosquito, Brazil has deployed thousands of municipal, state and federal workers, including soldiers, to scour cities for mosquito breeding grounds, fumigate and educate residents on the dangers of still and stagnant water, where the female insects lay their eggs.
On February 13, the government will deploy 220,000 troops in a one-day mobilisation to hand out leaflets and help identify potential trouble spots.
"All of the countries of Latin America and the Caribbean are trying to coordinate so that they take exactly the same measures to diminish not only the breeding grounds of this mosquito, which also carries dengue and chikungunya, but also to prevent it from spreading from one country to another," Al Jazeera's Lucia Newman reported from Santiago, the capital of Chile.
The Zika virus was first detected in 1947 in Uganda, and for decades, caused only mild diseases across Africa and equatorial Asia. But Chan noted that "the situation today is dramatically different".
There is no specific treatment or vaccine for Zika, which is related to Dengue. Scientists have struggled for years to develop a Dengue vaccine but have failed to create a viable shot so far.

A new virus is making its way through the Americas right now. It's not really deadly, but there is no cure or vaccination for it at this point. It has been proved by researchers of CDC that there were a few cases where pregnant women had the zika virus while they were pregnant, and their baby was born with defects such as microcephaly which is a condition where the baby's head is smaller than average. There is a way of preventing this virus and it's just making sure that mosquitoes can't get to you by wearing long clothing that cover any exposed body parts, and sleeping under mosquito net beds. The author of this article seems to be biased towards the Americans because they mention that though the virus was first detected in Uganda and spread across Africa and equatorial Asia, the situation NOW in the Americas is "dramatically different". I hope the CDC and WHO organizations are able to figure out the virus and its cure and vaccine. 

Al Jazeera and agencies. "Mosquito-borne Zika Virus 'spreading explosively'". Jan 29, 2016. Al Jazeera. Feb 1, 2016.