The hunt for a vaccine to prevent the Zika virus could take years, a top U.S. health chief said amid a worrying outbreak of the mosquito-borne illness blamed for birth defects, AFP reports.

There is no vaccine or treatment for Zika, which the World Health Organization said is "spreading explosively" through the Americas and may lead to as many as four million cases in the region.

Zika has been linked to a rise in cases of microcephaly -- abnormally small heads and brains -- in babies born to infected women in Brazil.

Anthony Fauci, the director of the National Institute for Allergy and Infectious Diseases (NIAID), said the U.S. government is working on two approaches toward a vaccine against Zika, based on research already done on related mosquito-borne viruses.

The first is a "DNA-based vaccine using a strategy very similar to what we employed for another flavivirus, the West Nile virus," he told reporters. Flaviviruses are generally transmitted by mosquitoes or ticks.

"Secondly, a live attenuated vaccine, building on similar and highly immunogenic approaches used for the closely related dengue virus," he added, according to AFP.

Hopes are high that a so-called Phase I clinical trial could begin later this year to test the safety and efficacy of a Zika vaccine in people, but Fauci cautioned that a finished product will take far longer.

"While these approaches are promising, it is important to understand that we will not have a widely available, safe and effective Zika vaccine this year and probably not even in the next few years," he said.