Former US first lady Barbara Bush died Tuesday at the age of 92, triggering widespread tributes for the matriarch of a Republican family that served at the top of American politics.

"A former First Lady of the United States of America and relentless proponent of family literacy, Barbara Pierce Bush passed away Tuesday, April 17, 2018 at the age of 92," said a statement from the office of George H.W. Bush. They were married for 73 years.

Barbara Bush has long been considered the rock at the centre of one of America's most prominent political families, as the wife of a president and the mother to another -- George W. Bush -- and to Jeb Bush, a former Florida governor and onetime presidential aspirant.

She first met her husband-to-be at the age of 16 when she was a schoolgirl and he was a student at an elite preparatory school in Massachusetts. They married in 1945 while he was on leave from wartime service as a naval officer. The couple had six children.

As the first lady, from 1989 to 1993, she embraced the cause of universal literacy and founded a foundation for family literacy.

As tributes poured in to honour the life of Barbara Bush, President Donald Trump hailed her as an "advocate of the American family."

"Amongst her greatest achievements was recognizing the importance of literacy as a fundamental family value that requires nurturing and protection," Trump said.

"She will be long remembered for her strong devotion to country and family, both of which she served unfailingly well."

- 'Force of nature' -

Her son Jeb wrote: "I'm exceptionally privileged to be the son of George Bush and the exceptionally gracious, gregarious, fun, funny, loving, tough, smart, graceful woman who was the force of nature known as Barbara Bush."

And his son George P. Bush tweeted: "I will miss you, Ganny—but know we will see you again."

"Barbara inspired us all to be the best version of ourselves," said Orrin Hatch, America's longest-serving Republican senator.

Madeleine Albright, who under Democratic president Bill Clinton was the first woman to serve as US secretary of state, remembered Bush as a "woman of incredible determination, wit and compassion who embodied America's best values."

Bush gained a reputation for toughness, wry humour and straight-speaking.

Asked in 2010 about former Alaska governor Sarah Palin -- who sought the vice presidency in 2008 -- she told an interviewer: "I sat next to her once, thought she was beautiful, and I think she's very happy in Alaska, and I hope she'll stay there."