The suspected link between the Zika virus and two neurological disorders, the birth defect microcephaly and Guillain-Barre syndrome, could be confirmed within weeks, the World Health Organisation (WHO) said Friday.

A sharp increase in microcephaly cases in Brazil triggered a global health emergency over the mosquito-borne virus, which previously had been viewed as causing only a relatively mild illness, and spurred a race to develop a vaccine, medicines and better diagnostic tests.

The WHO said U.S. government scientists and an Indian biotechnology firm were the front-runners in the vaccine effort, adding it would take at least 18 months to start large-scale clinical trials of potential preventative shots. The U.N. health agency also for the first time advised pregnant women to consider delaying travel to Zika-affected areas.

Brazil is at the center of the Zika outbreak that has spread to more than 30 countries. Researchers there are working to determine whether Zika has caused a big rise in cases of microcephaly, a birth defect in which babies are born with abnormally small heads and may have developmental problems.

Brazil's health ministry issued fresh figures Friday, reporting 4,314 suspected and confirmed cases of microcephaly, up from 4,074 cases Feb. 2. The ministry said it had confirmed 462 of those cases as microcephaly or other alterations to the central nervous system. Researchers have identified evidence of Zika infection in 41 of these cases, either in the baby or in the mother. But scientists have not confirmed Zika can cause microcephaly.

“It seems indeed that the link with Zika (and microcephaly) is becoming more and more probable, so I think that we need a few more weeks and a few more studies to have this straight,” Marie-Paule Kieny, WHO assistant director-general for health systems and innovation, told a news briefing in Geneva.

Studies of Zika-infected pregnant Latin American women who were due to deliver their babies soon should yield evidence, Kieny said, adding data also was coming from studies in French Polynesia and Cape Verde.

Kieny said Zika-hit areas also have experienced increased cases of the neurological disease Guillain-Barre, adding: "The direct causality has still to be demonstrated but the association in time and in location seems to be clear."

Guillain-Barre syndrome, in which the body's immune system attacks part of the nervous system, causes gradual weakness in the legs, arms and upper body and sometimes total paralysis.

In a statement, the WHO reiterated it was not recommending any general travel or trade restrictions related to the virus. But it added: “Women who are pregnant should discuss their travel plans with their healthcare provider and consider delaying travel to any area where locally acquired Zika infection is occurring.”

Brazil is set to host the Olympics in August in Rio de Janeiro, an event expected to draw hundreds of thousands of athletes, officials and spectators.

Many scientists are convinced the link between Zika and birth defects is real. Evidence of Zika in the brain of an aborted fetus, reported on Wednesday, added to the case.

Speaking at an American Association for the Advancement of Science news conference in Washington, Christopher Dye of the WHO reiterated the agency's strong suspicion.

“If we take all the information we have at the moment, the case for a causal link is quite strong,” Dye said. “We should now say that Zika is guilty until proven innocent.”

Kieny said two vaccine candidates seem to be more advanced: one from the U.S. National Institutes of Health (NIH) and one from the Indian company Bharat Biotech.

The NIH is working on a DNA-based vaccine that uses the same approach as one being developed for West Nile virus. India's Bharat said last week its experimental vaccine would start preclinical trials imminently in animals.

Overall, about 15 groups are working on Zika vaccines.

Kieny said new diagnostic test kits also were being developed rapidly and could be available within weeks.

Zika is predominantly spread by mosquito bites, but scientists are studying transmission by blood transfusions and sexual contact.

British health officials reported Zika was found in a British man’s semen two months after he was infected, suggesting the virus may linger in semen long after infection symptoms fade.

They said the 68-year-old man, infected in 2014 in French Polynesia, had low levels of the virus in initial blood tests. Subsequent tests of semen showed positive results at 27 days and 62 days after the start of Zika symptoms, with higher levels of the virus in the semen than the initial blood tests.

“Our data may indicate prolonged presence of virus in semen, which in turn could indicate a prolonged potential for sexual transmission” of this virus, the researchers from Public Health England and the National Institute for Health Research in Liverpool wrote in Emerging Infectious Diseases, a journal of the U.S. Centers for Disease Control and Prevention.

The WHO has advised women, particularly pregnant women, to protect themselves from mosquito bites in Zika-affected areas and to practice safe sex through the use of condoms.