"We make this call despite the widespread fatalism that the Rio 2016 Games are inevitable or 'too big to fail,' " the writers
said in the letter addressed to WHO Director-General Margaret Chan
. "Our greater concern is for global health. The Brazilian strain of Zika virus harms health in ways that science has not observed before."
The letter shows a growing gap within the medical field on what to do about the Games. On Thursday, Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, said, "There is no public health reason to cancel or delay the Olympics."
The CDC's current recommendation is that pregnant women should not travel to areas where the virus is spreading and that men with the virus who have pregnant partners should use condoms when having sex for the duration of the pregnancy.
"We're working closely with the USOC and Brazilian health authorities, and will update our guidance if needed," Frieden said in a statement in response to the new letter.
The WHO released a statement saying that based on its current assessment, "canceling or changing the location of the 2016 Olympics will not significantly alter the international spread of Zika virus."
The organization noted that Brazil is one of almost 60 countries that have reported continuing transmission of Zika by mosquitoes.
"People continue to travel between these countries and territories for a variety of reasons. The best way to reduce risk of disease is to follow public health travel advice," the WHO said.
The International Olympic Committee has said it has no plans to cancel or postpone the Games.
There have been calls in recent months for a delay or postponement of the Olympics, but what makes Friday's letter different is the scope and number of physicians, professors and bioethicists who signed it -- from Japan to South Africa, Norway to the United States.
The officials said the Zika virus has "more serious medical consequences than previously known" and has worsened in the Rio area despite widespread mosquito treatment programs.
"It is unethical to run the risk," the letter said. "It is therefore imperative that WHO conduct a fresh, evidence-based assessment of Zika and the Games, and its recommendations for travelers."
The Zika virus has been shown to cause microcephaly, a rare birth defect in which babies develop abnormally small heads and other neurological problems.
The Olympics are set to begin in just over two months, running from August 5 to August 21. More than 500,000 people are expected to travel to Brazil from around the world.
Olympic-related travel represents just 0.25% of the total 40 million travelers between the U.S. and countries where the Zika virus is circulating, according to the CDC.
Friday's letter was written by four prominent professors and had the backing of more than 100 well-known doctors and professors.
One of the letter's co-authors, Lee Igel, took issue with Frieden's comments that there is "no public health reason" to delay the Games.
"If you think that a mega-sports event in the midst of a major virus outbreak in a host city dealing with a turbulent economy, sitting on top of a turbulent political situation, sitting on top of a turbulent social condition, doesn't pose a significant public health issue, then, sure, 'On with Games,' " Igel wrote in an e-mail to CNN.
Igel, the co-director of New York University's Sports & Society program, said his biggest fear should the Games move forward is "how are human and financial resources going to be managed effectively given the reality of all things happening on the ground in Rio."
New York University bioethecist Art Caplan, another co-author, told CNN that there needs to be a two-day summit of independent experts to analyze the risks posed by keeping the Games in Brazil on the current time frame.
"Put it online and let the whole world watch," Caplan said. "What I don't like is when experts come out and say, 'There's not much reason to be worried. These Games should go on.'
"What I want to know: What are your arguments? What risks are we talking about? If something goes wrong, who's liable, and who's going to take the blame?"
Brazil has been at the epicenter of the Zika virus, with infectious disease experts descending on the hardest-hit areas to investigate why it's spreading
and why it has resulted in babies being born with microcephaly. They are also looking into the link between the virus and neurological disorders in fetuses, newborns, infants and adults. This includes trying to quantify the risk for pregnant women and others.
Igel said he and the others who signed onto the letter will continue "ringing the bell" about public health fears of the Games remaining in Brazil.
The U.S. Olympic Committee has told athletes to skip the Games if they're concerned about Zika.
Last week, USA Swimming announced the relocation of its pre-Olympic training camp from Puerto Rico to Atlanta because of concerns over the virus.
Dr. Ford Vox, a CNN contributor and physician who works in brain injury medicine with the Shepherd Center in Atlanta, said he signed the letter because he believes a delay or postponement is necessary.
"In my opinion, non-essential travel to Zika endemic areas should be deferred until the situation improves, and the Olympics are not essential," he said.