One man was killed and another was seriously injured when they encountered a cougar on Saturday while mountain biking in Washington state.

Authorities said the two men were on a morning bike ride in the foothills near North Bend when the attack occurred.

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The cougar ran into the woods but was later tracked and euthanised by officers from the Washington Department of Fish and Game, according to a tweet by the King County Sheriff's Department.

The 31-year-old survivor was taken to a hospital 48 kilometres west of Seattle.

He was listed as being in a serious condition but was alert and talking, Harborview Medical Centre said in a statement.

A search and rescue team has been dispatched to recover the body of the deceased man.

KIRO-TV reported the injured man called 911 shortly before 11am and shouted, "Can you hear me? Help!" and then the call hung up.

Authorities found the cougar standing over the body of the dead rider, the station reported.

A New Mexico man was killed by a mountain lion in 2008. KIRO-TV reported this was the first fatal attack by the animal in Washington State since 1924.

It was not immediately clear if the two victims were biking together or separately.

AP/ ABC