The director-general of the World Health Organisation warns that possible links between the Zika virus and babies with abnormally small heads are causing “extremely high” levels of concern.
Announcing that the WHO would be convening an emergency committee meeting on 1 February, Margaret Chan said the organisation would consider the international response to the outbreak.
The Zika virus, which is spread to people through mosquito bites, has been present in several African countries for decades. It usually causes only mild symptoms.
But since the first case was reported in Brazil in May 2015, there have been numerous reports of women who have had the virus giving birth to babies with defects, in particular microcephaly, where a baby is born with an unnaturally small head.
Dr Chan said on Thursday she expected to see birth defects related to the Zika virus in other countries too.
According to Dr Sylvain Aldighieri, of the Pan American Health Organisation, if the Zika virus spreads in the same way as the viral infection dengue, there could be as many as three to four million cases in the Americas in the next 12 months.
It is also feared that there may be a link with an increase in the number of people suffering Guillain-Barre syndrome, which can cause temporary paralysis.
Health officials in the USA said clinical trials on two possible Zika vaccines could begin by the end of 2016, but warned it would be several years before any vaccine was widely available.
Currently Zika virus is present in Brazil, Mexico, the Caribbean, Cape Verde, El Salvador, Guatemala, Honduras, Panama, Bolivia, Colombia, Ecuador, French Guiana, Guyana, Paraguay, Samoa, Suriname and Venezuela.
The WHO will be keen to act swiftly as following the 2014-5 outbreak of Ebola in Sierra Leone, Guinea and Liberia, it was criticised for its failure to respond fast enough to a crisis that went on to kill over 10,000 people.
A report into what had gone wrong found that the WHO did not have a culture of rapid decision-making, and did not adopt a proactive approach to emergencies, and it suggested a raft of changes.
With no treatement or vaccine against Zika, pregnant women are being advised to avoid travelling to affected areas where possible. Women who are considering getting pregnant should consult their doctors.
Other travellers should protect themselves against mosquito bites where possible.