U.S. President Barack Obama on Sunday mourned the passing of Nancy Reagan, the wife of late former President Ronald Reagan, saying she redefined the role of first lady.

The former first lady died Sunday of congestive heart failure at her home in Los Angeles at the age of 94, her spokeswoman Joanne Drake of the Ronald Reagan Presidential Library said in a statement. She will be buried at the library in Simi Valley, California, next to her husband who died in 2004, Drake said.

"Nancy Reagan once wrote that nothing could prepare you for living in the White House. She was right, of course. But we had a head start, because we were fortunate to benefit from her proud example, and her warm and generous advice," Obama and first lady Michelle Obama said in a statement.

"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.

Nancy Reagan, first lady from 1981 to 1989, was considered one of the most influential first ladies in U.S. history. During her time as first lady, she also led the anti-drug campaign "Just Say No."

After leaving the White House, she devoted herself to nursing her husband who was diagnosed with Alzheimer's in 1994 and died 10 years later. She later strongly advocated for stem cell research in the hope of finding a cure for the disease. (Yonhap)