The incident occurred on a remote stretch of the Jefferson River, where at least one otter swam up and attacked the women on Wednesday.

One victim was airlifted to hospital after sustaining severe wounds to her face and arms, and her condition is unknown.

The two other women also sustained injuries to their arms.

"It's just not something you run into very often," Jefferson County deputy sheriff James Everett said.

"Bears do it, moose too and occasionally a deer, but otters? That's not normal."

No effort has been made to catch or remove any of the animals, as it is believed the attack was done in defence.

Northern river otters are members of the weasel family. Eating a diet made up of primarily fish, they can reach the size of a small dog.

Morgan Jacobsen, who works with Montana Fish Wildlife and Parks, recommended that people "fight back, get away and get out of the water" if they are attacked by the creatures.

The incident follows a string of otter attacks on surfers in California last month, with the creatures gaining widespread attention after wrestling surfboards from their owners.