Former US first lady Rosalynn Carter has died at the age of 96.

She died peacefully with her family by her side at her home in Plains, Georgia, according to the Carter Center, a not for profit organisation founded by her husband and former president Jimmy Carter, 99.

They were married for 77 years.

Mrs Carter had been suffering from dementia.

“Rosalynn was my equal partner in everything I ever accomplished,” Mr Carter said.

“She gave me wise guidance and encouragement when I needed it. As long as Rosalynn was in the world, I always knew somebody loved and supported me.”

Unlike many previous first ladies, Rosalynn Carter sat in on cabinet meetings and represented her husband on foreign trips.

She was known by aides, privately, as “co-president” during Mr Carter’s single term of office from 1977-1981.

At one stage, she was forced to declare: “I am not running the government.”

Mrs Carter is survived by her children Jack, Chip, Jeff, and Amy, 11 grandchildren and 14 great-grandchildren.

One grandson died in 2015, the Carter Center said.

Chip Carter said his mother was a “great humanitarian” who will be “sorely missed not only by our family but by the many people who have better mental health care and access to resources for caregiving today”.