US rangers have found a human foot floating in a hot pool in Yellowstone national park, the park service said Friday, warning visitors to stay away from thermal waters.

The partial foot was inside a shoe in the Abyss Pool, one of the deepest hot springs in the park, whose temperature is around 140 Fahrenheit (60 Celsius).

"An investigation by Yellowstone National Park law enforcement officers is ongoing," the park said.

​"Evidence from the investigation thus far suggests that an incident involving one individual likely occurred on the morning of July 31, 2022, at Abyss Pool.

"Currently, the park believes there was no foul play."

Accidents are not unheard of in the thermal pools that dot the country's oldest national park.

In 2016 a young man died after slipping off a boardwalk and falling into a hot spring at the Norris Geyser Basin.

Last year two people had to be treated after being scalded by waters in the park.

"Visitors are reminded to stay on boardwalks and trails in thermal areas and exercise extreme caution around thermal features," the park service said.

"The ground in hydrothermal areas is fragile and thin, and there is scalding water just below the surface."

Yellowstone, which welcomed more than 4.8 million visitors last year, spreads across 3,500 square miles (9,000 square kilometers) of Wyoming, Idaho, and Montana.

It is home to thousands of geothermal features – hot springs, mudpots, steam vents, and about half the world's active geysers, including Old Faithful.

© Agence France-Presse