A cougar has been shot dead by the authorities in the US after mauling a mountain biker to death.

Another man is said to have been injured by the mountain lion, but his condition is not believed to be life-threatening.

The incident happened on Saturday in King County in Washington state, around 50 kilometres east of Seattle.

I am heading to the scene of the Cougar attack. One victim 40’s transported to HVH for treatment. Second victim found dead from attack. I will update with additional info when I arrive.

— KingcosoPIO (@kingcosoPIO) May 19, 2018

The two men are said to have biking down a remote trail when they encountered the cougar.

The animal lunged at one of the bikers, leaving him with scratches, before turning to the other man who was trying to escape.

According to local media, the first man managed to escape on his bike and cycled around two miles before getting mobile phone coverage and calling the authorities.

Officials arrived at the scene and found the cougar with the other man's body.

A spokesperson for the local sheriff's office told Reuters: "He or she, I don’t know if the cougar was a male or female, had the body of the victim down in his den."

An officer took a shot at the large cat, which fled into the nearby woods.

The big cat was later tracked down and killed by officials from the state's Department of Fish and Game.

Update: Fish and Wildlife has euthanized the cougar that is believed to be responsible for killing one man and attacking another earlier today.

— KingcosoPIO (@kingcosoPIO) May 19, 2018

The incident in Washington is believed to be only be the second fatal cougar attack in the state in nearly 100 years.

Cougars - also known as pumas - can be found in habitats across both North and South America.