One of the victims was left with dozens of stitches on her face and body in the "vicious and relentless" attack.

Jen Royce was tubing on the Jefferson River with two friends shortly after 8pm on 2 August when the otter appeared behind one of them and attacked.

Warning - this article includes images some readers may find distressing

"I didn't even get a chance to get the words, 'there is an otter behind you,' out of me before it attacked her," she posted on Facebook.

Ms Royce's face, arms, ears, hands, legs and ankle were all bitten by the otter as it attacked the group for about five minutes.

The three women eventually managed to get to the shore and the otter swam away.

"Without ANY exaggeration, God's honest truth, I did not think I was going to make it out of that river. I had no clue if my friends were going to make it out. But by the grace of God we did," the mother of three said.

"I was covered in blood and it just kept pouring out of my face and nose. It was cold. We were wet. It was dark."

Ms Royce said she was "extremely faint" as an ambulance struggled to locate the women in the remote area.

She was airlifted to hospital with severe bites to her face and arms, while the other women had superficial wounds.

Ms Royce was released from hospital a week after the incident and she reported her wounds are healing well.

A GoFundMe has been set up to help with her medical bills.

The Montana Fish, Wildlife and Parks has posted signs at several fishing sites in the area advising people of otter activity in the area.

"While attacks from otters are rare, otters can be protective of themselves and their young, especially at close distances," the department said.

"They give birth to their young in April and can later be seen with their young in the water during the summer. They may also be protective of food resources, especially when those resources are scarce."

Adult North American river otters weigh around 9kg (20 pounds) and are about 1.2m (47 inches) long.