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Award-winning US science fiction and fantasy author Ursula K Le Guin has died, aged 88, her family said.

The best-selling writer passed away on Monday at her home in Portland, Oregon, after a period of ill health.

Le Guin's books melded dragons and wizards with spaceships to tackle earth-bound problems of race, gender and class.

She wrote more than 20 novels and over 100 short stories that sold millions of copies around the world.

Her verified Twitter account posted a statement on Tuesday evening saying: "The family of Ursula K Le Guin is deeply saddened to announce her peaceful death yesterday afternoon."

She was best known for the Earthsea series, written for young adults, and her 1969 sci-fi classic The Left Hand of Darkness, set on a planet called Gethen, where everyone is ambisexual.

"I tend to avoid fiction about dysfunctional urban middle-class people written in the present tense," she once said.

"This makes it hard to find a new novel, sometimes."

In a career spanning more than half a century, she won a number of Nebula and Hugo science fiction and fantasy awards.

She also received the Newbery Medal, the top honour for US children's literature and the National Book Foundation Medal for Distinguished Contribution to American letters.

The US Library of Congress in 2000 designated her a Living Legend for her contribution to America's cultural heritage.

Reacting to news of her death, US horror author Stephen King paid tribute to Le Guin as "one of the greats".

"Not just a science fiction writer; a literary icon. Godspeed into the galaxy," he wrote on Twitter.

Other fellow authors and fans have been remembering the impact she made.

Le Guin was born Ursula Kroeber on 21 October 1929 in Berkeley, California.

She was educated at Radcliffe College, Massachusetts, and New York's Columbia University, becoming a Fulbright Fellow in 1953.

She became an expert in anthropology and was influenced by anarchist and Taoist thinking.

Her first novel, Rocannon's World, was published in 1966.

She married historian Charles Le Guin, and the couple had three children.