First lady Michelle Obama will attend the funeral on Friday of Nancy Reagan, who will be buried next to her husband, former President Ronald Reagan, at his presidential library in California, officials said on Monday.

Reagan will lie in repose at the Ronald Reagan Presidential Library on Wednesday and Thursday. A private funeral is planned for Friday, the Ronald Reagan Presidential Foundation said.

"First lady Michelle Obama will be attending Mrs. Reagan's funeral," a White House official said on Monday.

Reagan, the former actress who was fiercely protective of her husband through a Hollywood career, eight years in the White House, an assassination attempt and his Alzheimer's disease, died on Sunday at age 94.

The cause of death was congestive heart failure, said a spokeswoman for the Reagan presidential library. She died at her Los Angeles home.

In Washington, President Barack Obama ordered that US flags be flown at half-staff at the White House, public buildings, military posts and other facilities as a mark of respect, the White House said on Monday.

Reagan became one of the most influential first ladies in US history during her Republican husband's presidency from 1981 to 1989.

Her husband, who affectionately called her "Mommy," while she called him "Ronnie," died in 2004 after a long struggle with Alzheimer's, the progressive brain disorder that destroys memory.

Obama, a Democrat, and his wife, Michelle, said Nancy Reagan redefined the role of first lady.

"Nancy Reagan once wrote that nothing could prepare you for living in the White House," the Obamas wrote in a joint statement on Sunday. "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."