Less than three weeks before her 100th birthday, the actress reportedly died at her home on Friday.

Her agent and close friend Jeff Witjas told People magazine: "Even though Betty was about to be 100, I thought she would live forever."

She had a career spanning 80 years and had Emmy-winning roles on television sitcoms The Golden Girls and The Mary Tyler Moore Show.

White was still starring in a TV sitcom, Hot in Cleveland, at age 92 until it was cancelled in late 2014.

"Old age hasn't diminished her," the New York Times wrote in 2013. "It has given her a second wind."

Biden joins Hollywood actors in paying tribute

US President Joe Biden tweeted: "Betty White brought a smile to the lips of generations of Americans. She's a cultural icon who will be sorely missed. Jill and I are thinking of her family and all those who loved her this New Year's Eve."

First Lady Jill Biden said: "Who didn't love Betty White? We're so sad."

Hollywood comedy actor Ryan Reynolds tweeted to say White was "great at defying expectation".

He continued: "She managed to grow very old and somehow, not old enough. We'll miss you, Betty. Now you know the secret."

Star Trek legend George Takei said: "Our national treasure, Betty White, has passed just before her 100th birthday. Our Sue Ann Nivens, our beloved Rose Nylund, has joined the heavens to delight the stars with her inimitable style, humor, and charm. A great loss to us all. We shall miss her dearly."

Oscar-winning actress Viola Davis tweeted: "RIP Betty White! Man did I think you would live forever. You blew a huge hole in this world that will inspire generations. Rest in glorious peace….you've earned your wings."

US actor Henry Winkler tweeted to say he was finding it hard to "absorb" White's death and thanked her for her "warmth and activism".

Legally Blonde actress Reese Witherspoon tweeted: "So sad to hear about Betty White passing. I loved watching her characters that brought so much joy. Thank you, Betty, for making us all laugh!"

The Martin Luther King, Jr. Center paid tribute to White as a woman who was criticised for having a black tap dancer on her show in 1954.

White had replied "live with it" before the show was cancelled soon after.

The elderly actress with a naughty sense of humour

Towards the end of her career, White said her longevity was a result of good health, good fortune and loving her work.

"It's incredible that I'm still in this business and that you are still putting up with me," White said in an appearance at the 2018 Emmy Awards ceremony, where she was honoured for her long career.

White was not afraid to mock herself and throw out a joke about her sex life or a snarky crack that one would not expect from a sweet-smiling, white-haired elderly woman.

She was frequently asked if, after such a long career, there was anything she still wanted to do.

Her standard response was: "Robert Redford."

A pioneering career that began before the Second World War

White, who was born in Oak Park, Illinois, in 1922, started her entertainment career in radio in the late 1930s.

She made her TV debut singing on an experimental channel in Los Angeles in 1939.

After serving in the American Women's Voluntary Service, which helped the US effort during World War Two, she was a regular on Hollywood on Television, a daily five-hour live variety show, in 1949.

A few years later she became a pioneering woman in television by co-founding a production company and serving as a co-creator, producer and star of the 1950s sitcom Life With Elizabeth.

Through the 1960s and early 1970s White was seen regularly on television, hosting coverage of the annual Tournament of Rose Parade and appearing on game shows such as "Match Game" and "Password".

White reached a new level of success on The Mary Tyler Moore Show, playing the host of a home-making television show, the snide, lusty Sue Ann Nivens, whose credo was "a woman who does a good job in the kitchen is sure to reap her rewards in other parts of the house".

An award-winning actress and a principled supporter of animal welfare

The actress won best-supporting actress Emmys for the role in 1975 and 1976.

She won another Emmy in 1986 for The Golden Girls, a sitcom about four older women living together in Miami that featured an age demographic rarely highlighted on American television.

White also was nominated for an Emmy six other times for her portrayal of the widowed Rose Nylund, a sweet, naive and ditzy Midwesterner, on the show, which ran from 1985 to 1992 and was one of the top-rated series of its time.

After a less successful sequel to The Golden Girls came a series of small movie parts, talk-show appearances and one-off television roles, including one that won her an Emmy for a guest appearance on The John Larroquette Show.

By 2009 she was becoming ubiquitous with more frequent television appearances and a role in the Sandra Bullock film The Proposal.

In 2010, she appeared as the slightly unhinged professor June Bauer in the popular US sitcom Community.

White also worked for animal causes.

She once turned down a role in the Hollywood movie As Good as It Gets because of a scene in which a dog was thrown in a garbage chute.

White had no children and her third and final husband of 20 years, Allen Ludden, died in 1983.