Legendary actress Diane Keaton has died, aged 79.

Oscar-winning actress Diane Keaton has died at the age of 79, People magazine has confirmed.

In a statement, a spokesperson for Keaton confirmed that the beloved actress died in California on Saturday, October 11.

"There are no further details available at this time, and her family has asked for privacy in this moment of great sadness," the spokesperson added.

However, a source has told People that Keaton's health had "declined very suddenly", which they described as "heartbreaking for everyone who loved her".

"It was so unexpected, especially for someone with such strength and spirit," they added. "In her final months, she was surrounded only by her closest family, who chose to keep things very private. Even longtime friends weren't fully aware of what was happening."

Another source noted that until quite recently, Keaton would be spotted around her Brentwood, California neighbourhood, taking her dog for daily walks.

"Up until just a few months ago, she'd walk her dog every day. She was usually dressed the same, with a hat and her signature sunglasses regardless of the weather," they said.

"She was always very nice, funny and chatty. She'd talk to her dog like he was a person. She was eccentric and had this old-school Hollywood aura. She was very, very special."

It was this same eccentric, eclectic energy that made Keaton a star.

Following acting school and a stint on Broadway, Keaton became a household name in the 1970s after rising to fame in a string of Oscar-winning films.

In Francis Ford Coppola's 1972 film, The Godfather, Keaton got her breakthrough role as Kay Adams-Corleone, wife to Al Pacino's Michael Corleone.

She'd never read the book.

"I auditioned for The Godfather, not having ever read The Godfather or caring about The Godfather or anything, because all I was doing was auditioning. I needed to get work," Keaton told PEOPLE.

"I think the kindest thing that someone's ever done for me is that I got cast."

But it was her captivating, charming and comedic performance in Woody Allen's Annie Hall that first cemented her place in Hollywood history, and earned Keaton an Academy Award for Best Actress in 1978. Annie Hall was one of eight collaborations between Keaton and writer-director Woody Allen, who at one point considered Keaton his muse.

Allen wasn't Keaton's only frequent collaborator, however. She also made four movies with rom-com icon Nancy Meyers, including 2003's Something's Gotta Give. And of course, who could forget Keaton's other iconic, career-defining roles in films like The First Wives Club, the Book Club franchise, and her directorial debut, Hanging Up.

Keaton is survived by her two children, daughter Dexter and son Duke.

Image: Getty.