The World Health Organization has ruled out any change in timing or the location of the upcoming Olympic Games in Rio de Janeiro, shunning a call by doctors and scientists to shift the event over the Zika virus.

An open letter addressed to the global health body signed by 150 international doctors, scientists and researchers Friday had called for the August Games to be moved or delayed to help prevent the spread of Zika virus.

Holding the Games in Rio, the second worst affected city in Brazil, would be "irresponsible" and "unethical" and could risk spreading the virus to "poor, as-yet unaffected places" like Africa and South Asia, said the letter.

Zika, which can cause birth defects including a devastating syndrome known as microcephaly in which babies are born with unusually small heads and brains, can be introduced to a new region when a local mosquito picks it up from an infected human.

If it lives long enough, the mosquito then infects people it subsequently takes blood, starting a vicious cycle.

But the WHO said moving the Olympics would not have a major impact on the spread of Zika.

READ: No plans to move Rio Olympics, Brazil officials say

"Based on current assessment, cancelling or changing the location of the 2016 Olympics will not significantly alter the international spread of Zika virus," it said in a statement late Friday.

Brazilian authorities on Saturday also said the Games would proceed as planned, with the health ministry saying it would continue to follow the guidance of the WHO, which has deemed the risk of Zika infection in August -- the middle of winter in Brazil -- to be "minimal".

Nearly 1,300 babies have been born in Brazil with irreversible brain damage since the mosquito-borne Zika began to spread there last year.

The virus can also cause adult-onset neurological problems such as Guillain-Barre Syndrome, which can cause paralysis and death.

"An unnecessary risk is posed when 500,000 foreign tourists from all countries attend the Games, potentially acquire that strain, and return home to places where it can become endemic," experts from the United States, Britain, Canada, Norway, the Philippines, Japan, Brazil, South Africa, Turkey, and Lebanon, among others, had stated in their letter.

"Should that happen to poor, as-yet unaffected places (e.g., most of South Asia and Africa) the suffering can be great," it added.

The WHO and top health officials have called on those travelling to Brazil to take precautions against mosquito bites, and have said pregnant women should avoid areas where Zika is circulating, including Rio.

- Serena to go 'super protected' -

Tennis world number one Serena Williams admitted Saturday that the health dangers posed by the Zika virus were a concern, but said she still planned to compete in Rio.

"It's something that's been on my mind. I'm really just gonna have to go super protected," said the 34-year-old American, the defending Olympic champion.

Men's world number one Novak Djokovic said talk of moving or cancelling the Games was unrealistic.

"To cancel the Olympic Games is unthinkable, honestly. I mean, many athletes and people already planned in advance," the Serb tennis champion said, adding however that the impact of the outbreak on local people should not be underestimated.

"How about those people living there, you know? Not talking about them too much. So I think we have to look from different perspectives in order to make a right conclusion."

The Olympics and Paralympics, set for August 5 through September 18, "will take place during Brazil's wintertime when there are fewer active mosquitoes and the risk of being bitten is lower," the WHO said this month.

On Thursday, the top US public health official, Tom Frieden, said "there is no public health reason to cancel or delay the Olympic Games".

Given the big financial investments at stake, the open letter questioned whether the UN health agency was able to give a non-biased view of the situation.

It said the world body may not be properly considering the options, which include moving the Games to a place where Zika is not present, postponing them until Zika is under control, or cancelling them.

"WHO must revisit the question of Zika and postponing and/or moving the Games. 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," the letter said.

"Given the public health and ethical consequences, not doing so is irresponsible."