A curious formation that's getting attention in the US state of Maine is actually a spinning ice disc that is 91 metres wide.

The formation in the Presumpscot River in Greater Portland was widely shared on social media and has drawn comparisons to an alien spacecraft, a carousel and the moon.

The oddity drew a crowd that stood mesmerised by the slow-turning disc in Westbrook.

The ice formation is believed to have formed naturally in a part of the river where there's a circular current that creates a whirlpool effect, said Tina Radel, the city's marketing and communications manager.

Locals say they've seen smaller ice discs before, but nothing like this one, Radel said.

Rob Mitchell, who owns a nearby office building, spotted the alien-looking disc on Monday morning and immediately notified the city.

Officials say the disc is spinning slowly counter-clockwise, and has served as a big raft for ducks and other birds.

"The ducks were rotating on this big Lazy Susan," Mitchell said.

Australian Associated Press