The WHO's director-general said the spread of the mosquito-borne disease had gone from a mild threat to one of alarming proportions.
Marcos Espinal, an infectious disease expert at the WHO's Americas regional office, said: "We can expect 3 to 4 million cases of Zika virus disease", but he gave no time frame.
There is no vaccine or treatment for Zika, which is a close cousin of dengue and chikungunya and causes mild fever, rash and red eyes.
An estimated 80 per cent of people infected have no symptoms, making it difficult to know whether they have been infected.
WHO director-general Margaret Chan said the organisation would convene an emergency committee on Monday to help determine the level of the international response to an outbreak of the virus spreading from Brazil that is believed to be linked to severe birth defects.
"The level of alarm is extremely high," Ms Chan told WHO executive board members at a meeting in Geneva.
"[Zika] is now spreading explosively.
"As of today, cases have been reported in 23 countries and territories in the [Americas] region."
Five French citizens who were infected with the Zika virus while travelling abroad have returned home since the beginning of the year, authorities there said.
"None of the patients presented a severe form of the infection," the Health Ministry said in a statement, without naming the regions where the five had been travelling.
"There is no actual risk of transmission of the Zika virus in mainland [France]," the ministry added.
Three Canadians had also been infected with the virus while travelling abroad, Canadian health authorities said.
"We have no reason to believe that local transmission is a concern for Canadians," Canadian Health Minister Jane Philpott said, while urging travellers to take precautions.
Virus may also be transmitted through sex
As global health experts scramble to understand how the virus spreads and may lead to birth defects, two cases suggest it may be transmitted through sex, not just mosquitoes.
"There is one reported case of Zika virus through possible sexual transmission," US Centres for Disease Control and Prevention principal deputy director Anne Schuchat said.
"In another case, Zika virus was found in semen about two weeks after a man had symptoms with Zika virus infection, so that sort of gives you the biologic plausibility of spread."
But Ms Schuchat added: "The science is very clear to date that Zika virus is primarily transmitted to people through the bite of an infected mosquito. So that is really where we are putting the emphasis right now."
Brazil's Health Ministry said in November 2015 that Zika was linked to a foetal deformation known as microcephaly, in which infants are born with abnormally small heads.
Brazil has reported 3,893 suspected cases of microcephaly, the WHO said last week, more than 30 times more than in any year since 2010 and equivalent to 1 to 2 per cent of all newborns in the state of Pernambuco, one of the worst-hit areas.
Ms Chan said that while a direct causal relationship between Zika virus infection and birth malformations has not yet been established, it is "strongly suspected".
"The possible links, only recently suspected, have rapidly changed the risk profile of Zika from a mild threat to one of alarming proportions," she said.