The Rio 2016 organizing committee is worried about the rapid spread of the Zika virus in Brazil, but said on Tuesday it has not seen evidence of people canceling travel to the Olympics in August.

"Tickets have not been returned nor trips canceled," Rio 2016's communications director, Mario Andrada, told reporters.

The Brazilian government, however, is recommending that pregnant women stay away because the mosquito-borne virus has been linked to birth defects in thousands of newborns in Brazil, which could lead to fewer visitors for the Games in Rio de Janeiro.

President Dilma Rousseff's top aide said the Zika outbreak that has rapidly spread through the Americas is expected to cause a dip in tourism to countries where the virus is present.

"The risk is serious for pregnant women. For them (travel) is evidently not recommended," Rousseff's chief of staff, Jaques Wagner, told reporters on Monday. He said the risk to other visitors, especially athletes, is zero.

The World Health Organization on Monday declared the Zika virus an international health emergency that could infect as many as 4 million people in the Americas.

The virus has raised questions worldwide about whether pregnant women should avoid infected countries. Expectant mothers in the early months of gestation are most at risk of infection with Zika, which is thought to slow the growth of the brain of the fetus at a crucial stage of development.

WHO Director-General Margaret Chan said on Monday that delaying travel was something pregnant women "can consider" but added that if they needed to travel, they should take protective measures by covering up and using mosquito repellent.

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Brazil has reported some 3,700 suspected cases of babies born with abnormally small heads, a neurological condition called microcephaly.

Dr. Anthony Costello, WHO director for maternal, child and adolescent health, told reporters in Geneva on Tuesday that women should "make sure you are taking every precaution to prevent pregnancy if you're not planning it and you are traveling to the ... Olympics."

The Rio 2016 committee will follow travel guidelines issued by the WHO, which has not recommended any restriction on travel to Brazil, Andrada said. The committee believes colder and drier weather in August in Rio will reduce cases of Zika.

The virus was first identified in 1947 in rhesus monkeys in Uganda while scientists were studying yellow fever, according to the World Health Organization. It was identified in humans in 1952. Outbreaks of the disease have been discovered in Africa, Asia, the Americas and the Pacific.

Since it appeared in Brazil in May, Zika has spread to 24 countries and territories in the Americas. Top U.S. airlines are promising refunds for tickets to the region.