ATLANTA, United States (AP) — Former First Lady Rosalynn Carter, the closest adviser to Jimmy Carter during his one term as US president and their four decades thereafter as global humanitarians, has died at the age of 96.

The Carter Center said she died Sunday after living with dementia and suffering many months of declining health. The statement announcing her death said she "died peacefully, with family by her side" at 2:10 pm at her rural Georgia home of Plains.

"Rosalynn was my equal partner in everything I ever accomplished," Jimmy Carter said in the statement. "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."

The Carters were married for more than 77 years, forging what they both described as a "full partnership". Unlike many previous first ladies, Rosalynn sat in on Cabinet meetings, spoke out on controversial issues, and represented her husband on foreign trips. Aides to President Carter sometimes referred to her — privately — as "co-president".

"Rosalynn is my best friend ... the perfect extension of me, probably the most influential person in my life," Jimmy Carter told aides during their White House years, which spanned from 1977-1981.

Fiercely loyal and compassionate as well as politically astute, Rosalynn Carter prided herself on being an activist first lady, and no one doubted her behind-the-scenes influence. When her role in a highly publicised Cabinet shake-up became known, she was forced to declare publicly, "I am not running the Government".

Many presidential aides insisted that her political instincts were better than her husband's — they often enlisted her support for a project before they discussed it with the president. Her iron will, contrasted with her outwardly shy demeanour and a soft Southern accent, inspired Washington reporters to call her "the Steel Magnolia".

Both Carters said in their later years that Rosalynn had always been the more political of the two. After Jimmy Carter's landslide defeat in 1980 it was she, not the former president, who contemplated an implausible comeback, and years later she confessed to missing their life in Washington.

Jimmy Carter trusted her so much that in 1977, only months into his term, he sent her on a mission to Latin America to tell dictators he meant what he said about denying military aid and other support to violators of human rights.

She also had strong feelings about the style of the Carter White House. The Carters did not serve hard liquor at public functions, though Rosalynn did permit US wine. There were fewer evenings of ballroom dancing and more square dancing and picnics.

Throughout her husband's political career she chose mental health and problems of the elderly as her signature policy emphases. When the news media didn't cover those efforts as much as she believed was warranted, she criticised reporters for writing only about "sexy subjects".

As honorary chairwoman of the Presidential Commission on Mental Health she once testified before a Senate subcommittee, becoming the first first lady since Eleanor Roosevelt to address a congressional panel. She was back in Washington in 2007 to push Congress for improved mental health coverage, saying, "We've been working on this for so long, it finally seems to be in reach."

She said she developed her interest in mental health during her husband's campaigns for Georgia governor.

"I used to come home and say to Jimmy, 'Why are people telling me their problems?' And he said, 'Because you may be the only person they'll ever see who may be close to someone who can help them,' " she explained.

After Ronald Reagan won the 1980 election Rosalynn Carter seemed more visibly devastated than her husband. She initially had little interest in returning to the small town of Plains, Georgia, where they both were born, married and spent most of their lives.