American endurance swimmer Diana Nyad has abandoned her latest attempt to swim the 166 kilometers from Cuba to Florida.

Making the journey without the benefit of a shark cage, the 62-year-old Nyad had reached the halfway point despite suffering painful stings by Portuguese Man o' War jellyfish. But medics warned her that with toxins from the stings building up in her body, another sting could be life-threatening.

She set out from Havana on Friday in her third attempt to reach Key West across the treacherous Straits of Florida.

Nyad made her second cage-less attempt last month before an asthma attack forced her out of the water.

Had she completed the swim, she would have established a new record for open-water swimming without a shark cage.

Australian swimmer Susan Maroney, 22, became the first person to swim from Cuba to the United States in 1997, though with the use of a shark cage.

Nyad made her first attempt in 1978 when she was 28 years old, but ended it because of high winds and rough seas.