The World Health Organization on Saturday rejected a call from 150 health experts to consider postponing or moving the Rio Summer Olympics due to the Zika virus in hard-hit Brazil, arguing that the shift would make no significant difference to the spread of the virus.

The UN health agency, which declared the spread of Zika in the Americas a global emergency in February, said in a statement there is "no public health justification" for postponing or canceling the 2016 games, which run from Aug. 5-21.

Hundreds of thousands of people from around the world are expected to travel to Rio de Janeiro and other Brazilian destinations this summer to see some 10,000 athletes compete at the games.

In an open letter to the WHO director-general released Friday, experts from over two dozen countries in fields including public health, bioethics and pediatrics — among them former White House science adviser Dr. Philip Rubin — called for the Rio games to be delayed or relocated, though not cancelled, "in the name of public health."

Brazilian health authorities are convinced that this baby's head condition, known as microcephaly, is related to the Zika virus that infected her mother during pregnancy. (Felipe Dana/Associated Press)

Friday's letter cited recent scientific evidence that the Zika virus causes severe birth defects, most notably babies born with abnormally small heads. In adults, it can cause neurological problems, including a rare syndrome that can be fatal or result in temporary paralysis.

The authors also noted that despite increased efforts to wipe out the mosquitoes that spread Zika, the number of infections in Rio de Janeiro have gone up rather than down.

Pregnant women should still not travel to Zika areas

Several public health academics have previously warned that having so many people travel to the games in Brazil will inevitably lead to the births of more brain-damaged babies and speed up the virus' global spread.

WHO, however, said "based on current assessment, cancelling or changing the location of the 2016 Olympics will not significantly alter the international spread of Zika virus." It argued that Brazil is only one of dozens of countries where mosquitoes transmit the Zika virus and says "people continue to travel between these countries and territories for a variety of reasons."

"Based on the current assessment of the Zika virus circulating in almost 60 countries globally and 39 in the Americas, there is no public health justification for postponing or cancelling the games," it said. "WHO will continue to monitor the situation and update our advice as necessary."

The agency noted its existing advice urging pregnant women not to travel to areas with Zika transmission, among other recommendations, and says other travellers should avoid the poor, overcrowded parts of Rio.

One of the letter's authors wasn't impressed by the UN agency's arguments.

"The WHO's response is absolutely fanciful," said Amir Attaran, a professor at the University of Ottawa and one of the letter's authors. He called WHO's argument that Zika is already being transmitted by mosquitoes in up to 60 countries "a scientific half-truth."

"They're avoiding the question of `Is it Brazilian Zika in other countries?"' he said.

A municipal health worker sprays insecticide in a junkyard to combat the Aedes aegypti mosquito that transmits the Zika virus, in Joao Pessoa, Brazil. (Andre Penner/Associated Press)

Friday's letter pointed to the particularly high risks from the Zika virus strain seen in Brazil, which has by far the most Zika cases in the world and the most brain-damaged Zika babies.

WHO emergency response chief Dr. Bruce Aylward told the BBC the agency was "maintaining a careful ongoing risk assessment as new information becomes available about this disease."

"We need to do a better job, perhaps, of communicating everything that's being done," he said.

The WHO statement didn't address the concerns in Friday's letter that the UN health agency was rejecting alternatives to Rio because it had a conflict of interest due to its long relationship with the International Olympic Committee.

The health experts called that relationship "overly close," but the IOC dismissed that characterization, saying it "does not currently have" a memorandum of understanding with the World Health Organization. The last one, it added, "outlined co-operation between the two organizations to promote physical activity to fight strokes, heart attacks, diabetes and obesity."

Olympics risk 'drop in the ocean'

Not all scientists agree that the Rio Olympics pose a problem.

"We live in an incredibly interconnected world. Global travel and trade are daily activities that offer the Zika virus an opportunity to spread," said Jonathan Ball, a professor of molecular virology at the University of Nottingham. "By comparison to these routine activities, the increased risk that the Olympics poses is a drop in the ocean."

He added, though, that people should still take care to both avoid mosquito bites and avoid places undergoing Zika outbreaks if they are or might become pregnant.

Concerns over Zika prompted USA Swimming to move its pre-Olympic training camp from Puerto Rico to Atlanta. Major League Baseball also scrapped games that were going to be held in San Juan, Puerto Rico.

No Olympic Games have ever been moved from their host city due to medical concerns, but in 2003, FIFA decided to switch the Women's World Cup soccer tournament on very short notice from China to the United States due to the threat posed by the respiratory virus SARS.