A tropical disease specialist will monitor sailors competing in Rio's polluted waters ahead of the 2016 Olympics, a top official said Friday, adding he was also worried about boats hitting garbage.

Top sailors from around the world begin a dress rehearsal Saturday for next year's Olympics in the scenic but filthy waters of Guanabara Bay in the heart of Rio de Janeiro.

But preparations for the August 15-22 test event, as well as the 2016 Games themselves, have been overshadowed by potential health risks from raw sewage and debris.

Despite repeated assurances from Brazilian authorities that the bay is safe, Alistair Fox, head of events with the International Sailing Federation, said the sport's governing body is taking the problem seriously.

"We've specifically appointed a member of our medical mission, a trained doctor in tropical medicine to be here," he told a press conference at the Rio harbor where Olympic sailing is based.

"The other area obviously we're very concerned about is objects in the water that could affect the fairness of racing," he said.

A sailing federation official will be monitoring the water with clean-up boats and from a helicopter every day of the test event, he said.

"We need to be able to make the right decisions."

Fox said there was "a lot of pressure on the (Rio) government" to do more about the pollution, "which is good."

Huge amounts of raw sewage from the Rio urban area currently pour into the bay, along with floating garbage, especially after rain.

Officials say they will not be able to meet their original goal of stopping 80 percent of the pollution by start of the Olympics.