GENEVA » Sexual transmission of the Zika virus is more common than previously thought, the World Health Organization said today, citing reports from several countries.

After a meeting of its emergency committee today, the U.N. health agency also said there is increasing evidence that a spike in disturbing birth defects and neurological problems are caused by Zika, which is mostly spread by mosquito bites. When WHO declared the explosive outbreak in the Americas to be a global emergency last month, it said that the evidence that Zika was responsible was only circumstantial.

WHO Director-General Dr. Margaret Chan said “reports and investigations in several countries strongly suggest that sexual transmission of the virus is more common than previously assumed.” The U.S. is investigating more than a dozen possible cases of Zika in people who may have been infected through sex.

Dr. Bruce Aylward, who is directing WHO’s response to Zika, said the cases seen so far of sexual transmission of Zika have been from men to women. He doubted sex would play an important role in the disease’s spread.

“The mosquito is undoubtedly still the main driver of transmission,” he said.

Chan also said nine countries have now reported increasing cases of Guillain-Barre syndrome, a rare condition that may be linked to Zika and can cause temporary paralysis and death in people of all ages. She said that problems connected to Zika, including Guillain-Barre syndrome, are now being seen not just in women of child-bearing age, but children, teenagers and older adults.

Zika is also now spreading to new countries, WHO said. It noted local transmission has now been reported in 31 countries across Latin America and the Caribbean.

“All of this news is alarming,” Chan said.

Despite the lack of definitive evidence proving that Zika causes birth defects and neurological problems, Chan said officials shouldn’t wait for definitive scientific proof before making recommendations.

“Microcephaly is now only one of several documented birth abnormalities associated with Zika infection during pregnancy,” she said, adding that it can cause growth problems, injuries to the central nervous system and fetal death.

WHO’s emergency committee called for “intensified” research into the relationship between new clusters of babies born with abnormally small heads and other neurological disorders. It said particular attention should be given to studying the genetics of the different Zika virus strains and establishing studies to determine if there is a causal relationship.

The agency also noted it was important to continue studying whether other factors could be responsible for the jump in microcephaly and neurological problems, including whether several causes combined might be to blame. Aylward explained that scientists were focusing on diseases as the main suspect, noting the evidence seems “really compelling that you’re looking at an infectious process here.”

So far, cases of babies born with small, deformed heads linked to Zika have only been confirmed in Brazil and French Polynesia, though officials say they expect reports from other countries once the virus has been circulating there long enough to affect pregnant women. Colombia has reported several suspected cases of microcephaly.

“Women who are pregnant in affected countries or travel to these countries are understandably deeply worried,” Chan said.

WHO recommends pregnant women avoid travel to areas with ongoing Zika outbreaks and that if their partners travel to affected countries, they should practice safe sex or abstain from sex for the duration of their pregnancy.

Cheng reported from London.