The WHO says women planning to travel to areas where the virus is circulating should consult a healthcare provider before travelling and on return.
The head of the WHO, Margaret Chan, says the situation is extremely worrying. "The explosive threat of Zika virus, to new geographical areas, with little population immunity is another cause for concern, especially given the possible link between infection during pregnancy and babies born with small heads."
Zika can be transmitted though blood, and has also been isolated in human semen.
The virus is native to Africa and was first found to be spreading in the Americas in Brazil in May last year.
The lack of any natural immunity in the Americas is thought to be helping the infection to spread rapidly.
It comes as health authorities in Australia are advising pregnant women to consider deferring travel to countries affected by the mosquito-transmitted Zika (ZEE-kuh) virus: Brazil, Paraguay, the Solomon Islands, New Caledonia and the Cook Islands.
Within Australia, only the mosquito that transmits yellow fever, found in north east Queensland, could carry the Zika virus.
Mosquito Researcher from Sydney University, Dr Cameron Webb has advised caution.
The problem is, is that when you have the yellow fever mosquite locally, if a traveller comes into the country carrying and are infected with one of these viruses, they can be bitten by local mosquitos that can then go on and bite locals, kicking off a local outbreak of one of these mosquito-borne diseases.
