WASHINGTON (AFP) - US President Barack Obama and his wife Michelle praised former first lady Nancy Reagan, who died Sunday at age 94, as a "proud example".

The Obamas said Reagan transformed the role of first lady during her 1981-1989 White House tenure, and later became an advocate for millions suffering from Alzheimer's disease like her husband, president Ronald Reagan, who died in 2004.

Extending condolences to her children and grandchildren, they said "we remain grateful for Nancy Reagan's life, thankful for her guidance, and prayerful that she and her beloved husband are together again".

The Obamas said they were "fortunate to benefit from her proud example, and her warm and generous advice".

"Our former first lady redefined the role in her time here.

"Later, in her long goodbye with President Reagan, she became a voice on behalf of millions of families going through the depleting, aching reality of Alzheimer's, and took on a new role, as advocate, on behalf of treatments that hold the potential and the promise to improve and save lives," they said in a statement.

Former first lady Barbara Bush said she and her husband, former President George H. W. Bush, who was vice president under Reagan, took comfort in knowing Nancy Reagan would be reunited with her husband, the late president.

The Hollywood glitterati weighed in on social media, many of them grieving the passing of an icon they remembered having grown up in the Reagan era.

"I sat near #Nancy Reagan once and felt like a teenager seeing one of my idols. She was a BOSS," wrote actress Elizabeth Banks of The Hunger Games fame.

Arnold Schwarzenegger, who like Ronald Reagan rode his Hollywood fame to the governor's office in California, said on Twitter that Nancy Reagan was "one of my heroes".

"She served as first lady with unbelievable power, class and grace and left her mark on the world," he continued. "She's with her Ronnie now, but those of us she left behind will miss her dearly."