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

After a meeting of its emergency committee on Tuesday, the UN health agency also said there is increasing evidence that a spike in birth defects and neurological problems is 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 it only had circumstantial evidence linking Zika to the health effects.

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 United States is investigating more than a dozen possible cases of Zika in people who may have been infected through sex.

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

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

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 it can cause injuries to the central nervous system and death.

Associated Press