LONDON (AP) — Athletes and visitors heading to the Rio de Janeiro Olympics should avoid poor and overcrowded parts of the city to minimize their chances of catching the Zika virus, The World Health Organization said Thursday.

The U.N. health agency also restated the advice it has been giving for months, warning pregnant women not to travel to Zika-hit areas. It also said that since the mosquito-borne disease can also be spread sexually, pregnant women should abstain from sex or practice safe sex with anyone who has recently returned from areas with outbreaks.

WHO declared the explosive spread of Zika in the Americas to be a global emergency in February and the virus has now been proven to cause a range of severe birth defects, including brain-damaged babies born with abnormally small heads and a rare neurological disorder that can cause temporary paralysis and is sometimes fatal.

Earlier this week, a Canadian professor called for the Olympics to be postponed or moved because of the epidemic, arguing that holding the Rio Games would result in the avoidable birth of malformed babies, as well as potentially sparking new outbreaks worldwide.

Speaking Thursday, he said the WHO advice was entirely inadequate.

"WHO has a moral and scientific duty to prevent these games from going ahead as scheduled," Amir Attaran, a public health specialist at the University of Ottawa, told The Associated Press. He questioned the utility of the advice to "avoid visiting impoverished and overcrowded areas in cities and towns with no piped water and poor sanitation."

"That's half of Rio," Attaran said. "They might as well just tell people not to go."

Other experts welcomed the statement.

"It's good to see that WHO has broken its silence on this question," said Suerie Moon, a research director at Harvard University's School of Public Health. She said the recommendations would carry more weight had WHO convened its independent group of experts to consider the issue of whether the Olympics should be moved or postponed.

WHO spokesman Christian Lindmeier said it was possible the agency might hold a meeting of its Zika experts in the coming weeks, but such a gathering would probably not specifically address the Olympics.

Lindmeier said WHO's advice might evolve if there is new information to consider.

WHO said that because the Olympics will take place during Brazil's winter, there will be fewer mosquitoes and the risk of being bitten will be lower.

Attaran pointed out that cases of dengue — spread by the same mosquitoes that spread Zika — have jumped sixfold this year despite Brazil's aggressive efforts to wipe out the insects, adding there is no evidence Zika transmission will disappear by the time the games start in August.