A mysterious oil slick was spotted in Rio de Janeiro's Guanabara Bay on Friday morning, the same body of water that will host Olympic sailing competitions when the Games come to the "Marvellous City" next summer.

It was not clear what caused the large, dark oil stain, but local reports indicated environmental authorities had gone to the area to look into where the dark cloud had come from.

Water quality in the picturesque bay, and other bodies of water to host Olympic events, has been under close scrutiny after studies revealed that waters to be used for events have pollution levels equivalent to raw sewage.

The cleaning of Guanabara Bay was a key part of Rio's bid and has long been a goal of successive local governments.

Hundreds of millions of dollars have already been spent, but the water remains fetid, with Olympic sailors who visited the city for test events complaining of floating sofas and animal carcasses in the water.

When it bid to host the Games, Rio said it would cut the amount of raw sewage flowing into the bay by 80 percent, but has since admitted that is unlikely.

Biologists last year said rivers leading into the bay contained a super bacteria that is resistant to antibiotics and can cause urinary, gastrointestinal and pulmonary infections.