August's Olympic Games in Rio de Janeiro should be postponed or moved to avoid intensifying the spread of the Zika virus, according to a letter signed by 151 leading scientists.
The World Health Organization (WHO) disagreed, saying Friday that the event would "not significantly alter" the spread of the mosquito-borne virus, which has been linked with microcephaly in babies born to affected mothers.
Zika only causes harmless flu symptoms in adults but in February the WHO declared the virus a public health emergency of international concern.
In March, it said that Latin America could be faced with thousands of cases of microcephaly, in which the brain does not properly develop and a baby's head is significantly smaller than normal. The outbreak has hit Brazil hardest.
The scientists calling for an Olympics postponement come from famous universities such as Oxford, Harvard and Yale.
"WHO must revisit the question of Zika and postponing and/or moving the Games," they said in an open letter to the WHO director general, Margaret Chan.
"We recommend that WHO convene an independent group to advise it and the IOC in a transparent, evidence-based process in which science, public health, and the spirit of sport come first.
"Given the public health and ethical consequences, not doing so is irresponsible," the letter also said, adding that half a million people are expected to travel to the Games in Rio.
The International Olympic Committee (IOC) has said the Games' schedule will not be changed despite concerns from several athletes.
One of the world's top golfers, Rory McIlroy, said this week he may skip the Games because of fears over the virus while fellow player Vijay Singh has already announced he will not attend.
Australia's Olympians will be given extra protection for the Games, in the form of "Zika-proof condoms," while South Korea's Olympic outfits have been made especially long to hide skin.
The Games, which run from August 5-21, could not realistically be transferred elsewhere this year such is the scale of the event.
There have also been worries expressed by sailors and open water swimmers about polluted, sewage-filled water in Rio's Guanabara Bay but Brazilian officials have said they are doing everything they can to address the matter.