Companies and scientists are racing to create a Zika vaccine as concern grows over the mosquito-borne virus that has been linked to severe birth defects and is spreading quickly through the Americas.

Zika is now present in 23 countries and territories in the Americas. Brazil, the hardest-hit country, has reported around 3,700 cases of the devastating birth defect called microcephaly that are strongly suspected to be related to Zika.

The Geneva-based World Health Organization (WHO), stung by criticism that it reacted too slowly to West Africa's Ebola epidemic, convenes an emergency meeting on Monday to help determine its response to the spread of the virus.

On Thursday, the WHO said as many as 4 million people in the Americas may become infected by Zika, adding urgency to the research efforts. Vaccine developers made clear a vaccine for widespread public use is at least months, if not years, away.

The closest prospect may be from a consortium including drugmaker Inovio Pharmaceuticals Inc that could have a vaccine ready for emergency use before year-end, according to one of its lead developers.

Many other drug makers also eying to have the vaccine ready by the end of the year.

Zika had been viewed as a relatively mild illness until Brazilian health officials identified it as a matter of concern for pregnant women. While a direct causal relationship has not been established, scientists strongly suspect a link between Zika and thousands of children born in Brazil with abnormally small heads, brain defects and impaired vision.

There is no treatment for Zika infection.

Brazilian President Dilma Rousseff vowed Friday to "win the war" against the Zika virus. US President Barack Obama spoke on Friday with Rousseff about the spread of the virus, the White House said.

Zika has hit Brazil just as it prepares to host the Olympic Games in Rio de Janeiro on Aug 5-21, an event that draws hundreds of thousands of athletes, team officials and spectators.