A new pollution problem has surfaced in Guanabara Bay, the venue for sailing at the Rio de Janeiro Olympics.

Sailors complained Monday about an oil slick that turned white boats brown with crews in the water practicing for the Olympics, which begin on Aug. 5.

"We've never seen anything like this. It was all over the place," said Finnish sailor Camilla Cedercreutz. "There was no way you could avoid it."

Cedercreutz said the slick filled part of the bay on Sunday, staining her boat from bow to stern.

"This is only our second time in Rio," said Cedercreutz. "We've heard it was really bad. You get mad because it shouldn't be like this anywhere. It shouldn't be this dirty. But there's nothing we can do about it."

'A lot of dead fish'

Cedercreutz's sailing partner Noora Ruskola said other sailors told her: "Your boat looks like a toilet."

Guanabara Bay is severely polluted, filled with bacteria and viruses. However, sailors have less frequently complained about industrial pollution in the giant bay.

Spanish sailor Jordi Xammar, who will compete in the 470 class, said he saw the slick "and tried to avoid it."

"The boats were completely brown," he said. "But the worst thing was we saw a lot of dead fish."

Xammar said this is his fourth time in Rio, and he's seen the water "improve a bit. It was yellow-green last year."

Plenty of pathogens?

It's yet another issue added to a long list of problems confronting South America's first Olympic Games: the Zika virus, rising crime and violence, budget cuts and lacklustre ticket sales.

The sporting events will also begin with Brazilian President Dilma Rousseff facing an impeachment trial and Brazil mired in its deepest recession in decades.​

Rio organizing committee officials say the venue is safe, although independent studies by The Associated Press show high level of pathogens — microorganisms that can cause disease — in waters that Rio is using for sailing, rowing, canoeing and open-water swimming.

World Sailing, the governing body of the sport, said Monday it was "not in a position to comment."