The UN health agency has revealed more details about the Zika virus following a meeting of its emergency committee.

The sexual transmission of the Zika virus is more common than previously thought, the World Health Organization has said, 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 disturbing birth defects is caused by Zika, which is mostly spread by mosquito bites.

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".

She said nine countries have now reported increasing cases of Guillain-Barre syndrome, a rare condition that can cause temporary paralysis and death, in people beyond women of child-bearing age, including children, teenagers and older adults.

"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.

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

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

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.

The agency last month said the explosive outbreak in the Americas constitutes a global emergency.