The WHO sent teams of senior scientists to Brazil four times "to gather first-hand data on the current situation and assess the level of risk to the large number of athletes and spectators expected to attend the Olympic Summer Games," Director-General Margaret Chan wrote in a letter dated June 1.
She was responding to a request by US Senator Jeanne Shaheen to evaluate the public health hazards of holding the Games in August.
"Given the current level of international concern, I have decided to ask members of the Zika Emergency Committee to examine the risks of holding the Olympic Summer Games as currently scheduled," Chan said.
The WHO chief said the experts were due to meet "shortly," and vowed to post their advice online "immediately."
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"The Olympic Games draw athletes and spectators from every corner of the globe and it's important that we understand the global health implications," Shaheen said in a statement after receiving Chan's letter.
Experts say Zika is to blame for a surge in cases in Latin America of microcephaly - a serious birth defect in which babies are born with unusually small heads and brains.
The exact date of the meeting should be published by the end of next week, spokeswoman Nyka Alexander told AFP in an email.
She pointed out that the experts' job was to "provide public health advice and technical guidance to the government of Brazil and the Rio 2016 Local Organising Committee on the public health risks of hosting a mass gathering such as the Olympics, and the recommended health systems that should be in place to host a safe and healthy event."
The WHO had previously rejected a call from more than 200 international doctors to change the timing or location of the Rio Games, saying shifting the Games would not substantially alter the risks of Zika spreading globally.
But concerns have been mounting since host country Brazil has by far been the hardest-hit since Zika began spreading in South America last year, with nearly 1,300 babies having been born there with irreversible brain damage since then.
The virus, which is mainly spread by two species of Aedes mosquito, but also through sexual contact, has also been linked to Guillain-Barre Syndrome, a rare but potentially fatal neurological disorder.