Scientists and private developers are doing all they can to develop a vaccine for the Zika virus.

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 World Health Organisation (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 four million people in the Americas may become infected by Zika.

However, vaccine developers have 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 that could have a vaccine ready for emergency use before year-end, according to one of its lead developers.

Canadian scientist Gary Kobinger says the first stage of testing on humans could begin as early as August.

If successful, the vaccine might be used during a public health emergency by October or November, said Kobinger, who helped develop a trial vaccine for the Ebola virus.

Privately owned vaccine developer Hawaii Biotech said it began a formal program to test a Zika vaccine last autumn as the virus started to gain traction in Brazil, although it has no timetable yet for clinical trials.

"Right now, we are in the pre-clinical stage, as I suspect everyone is," chief executive Elliot Parks said.

Another private vaccine developer, Replikins, said it was preparing to start animal studies on a Zika vaccine in the next 10 days.

It says data from the trials on mice and rabbits would likely be out in the next couple of months.

Current efforts to combat Zika are focused on protecting people from being bitten and on eradicating mosquitoes, a tough task in many parts of Latin America, where people live in poverty and there are many breeding grounds for the insect.

Brazilian President Dilma Rousseff said tests for the development of a vaccine would begin next week at the Butantan Institute, one of Brazil's leading biomedical research centres in Sao Paulo.

US President Barack Obama spoke on Friday with Rousseff about the spread of the virus, the White House said.

"The leaders agreed on the importance of collaborative efforts to deepen our knowledge, advance research and accelerate work to develop better vaccines and other technologies to control the virus," the White House said in a statement.