A curious formation attracting attention in the US is actually a spinning ice disc that is roughly 91 metres wide.

The formation in the Presumpscot River in Greater Portland has drawn comparisons to an alien spacecraft, a carousel and the moon.

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

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

Locals said they have seen smaller ice discs before but nothing like this one, Ms Radel said.

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

Officials said 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,” Mr Mitchell said.