A rare attack by a river otter in southern Montana injured three women floating on inner tubes and inflicted wounds serious enough that one victim had to be airlifted to a hospital, authorities said Thursday.Related video above: Otter steals surf boardThe attack happened near the town of Cardwell on a remote stretch of the Jefferson River, a tributary of the Missouri River that's popular with anglers and recreational floaters.At least one otter swam up to the adult women at about 8:15 p.m. Wednesday and attacked them, said Morgan Jacobsen with Montana Fish Wildlife and Parks. The women were able to get to shore, where one of them called 911, he said.One woman's wounds, on her face and arms, were so severe that the helicopter was used to fly her out, Jefferson County Undersheriff James Everett said. Her condition Thursday was unknown. The others had injuries to their arms.“It's just not something you run into very often," Everett said. "Bears do it, moose too and occasionally a deer, but otters? That's not normal.”Jacobsen said one of the women saw two otters beforehand but it was unclear how many were involved in the attack.Northern river otters are members of the weasel family and can reach up to 20 pounds — as heavy as a small dog — and up to 47 inches long. They primarily eat fish.They can use their teeth and claws to bite and scratch, Jacobsen said.“If folks are attacked by an otter, our recommended response is to fight back, get away and get out of the water,” he said.Warning signs were posted at access points along the Jefferson River in the area of the attack.No otters have been seen there since Wednesday and there will be no efforts to catch or remove any of the animals because it's believed to have been a defensive attack, he said.Two years ago a 12-year-old boy on an inner tube was attacked but not seriously hurt by an otter on Montana's Big Hole River.Last month in California, a sea otter gained widespread attention for aggressively wrestling surfboards from surfers off the coast of Santa Cruz.