GENEVA (AP) -- The World Health Organization's chief says the agency is increasingly worried about the Zika virus, even though it does not recommend canceling or postponing the upcoming Rio de Janeiro Summer Olympics.

In a briefing Tuesday in Geneva, Dr. Margaret Chan said "the more we learn about Zika, the more worried we get about it."

Chan noted that although Zika has been around for decades, it is only recently that the virus has been proven to cause severe birth defects and neurological problems.

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In February, WHO declared the explosive outbreak to be a global health emergency. Zika has now spread to nearly 60 countries.

Some experts have called for this year's Olympics to be moved or delayed to prevent the avoidable birth of brain-damaged babies. The games run Aug 5-21.