The Rio 2016 organising committee is worried about the rapid spread of the Zika virus in Brazil, but says it has not seen evidence of people cancelling travel to the Olympics in August.

"Tickets have not been returned nor trips cancelled," Rio 2016's communications director, Mario Andrada, said on Tuesday.

New Zealand's Lydia Ko, the women's world No.1 golfer, will continue to monitor news of the mosquito-borne Zika virus sweeping across South America before the Olympics.

The New Zealander didn't discuss a possible Games pullout and her concerns were for the people in the 23 countries and territories in the Americas that have been hit by the virus.

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, said 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.

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 travelling to the ... Olympics."

Since it appeared in Brazil in May, Zika has spread to 24 countries and territories in the Americas.

Top US airlines are promising refunds for tickets to the region.