The agency convened an emergency meeting of independent experts on Monday to assess the outbreak after noting a suspicious link between Zika's arrival in Brazil last year and a surge in the number of babies born with abnormally small heads.
Although WHO Director-General Dr Margaret Chan said there was no definitive proof that the Zika virus, spread by mosquitoes, is responsible for the birth defects, she acknowledged on Thursday that "the level of alarm is extremely high."
The last such public health emergency was declared for the devastating 2014 Ebola outbreak in West Africa, which killed more than 11,000 people.
WHO estimates there could be up to 4 million cases of Zika in the Americas in the next year.
The World Health Organisation has begun a crisis meeting considering whether the explosive spread of the mosquito-borne Zika virus - which is linked to birth defects in the Americas - should be declared a global health emergency.
"It is important to understand, there are several measures pregnant women can take," Chan said. "If you can delay travel and it does not affect your other family commitments, it is something they can consider.
"If they need to travel, they can get advice from their physician and take personal protective measures, like wearing long sleeves and shirts and pants and use mosquito repellent."
The closed-door teleconference meeting of experts is considering whether international efforts to fight the outbreak should be immediately ramped up, WHO spokesman Christian Lindmeier said. The UN. health agency last declared an emergency over the 2014 Ebola outbreak in West Africa. A similar declaration was made for polio the year before.
Such 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.