A French water charity sounded the alarm Monday about pollution in the river Seine, which is set to be used for swimming during the Olympic Games.

Surfrider Foundation said it had analysed six months of tests undertaken by a laboratory and had concluded that the river water remained polluted and potentially dangerous.

In an open letter, the Biarritz-based charity said it "wanted to share with stakeholders its rising concerns about the quality of the Seine but also the risks faced by athletes moving in contaminated water."

Paris authorities are in a race against time to clean up the Seine before the start of the Olympics on July 26, with the famed waterway set to play a starring role during the Games.