Michelle Obama Visits Nelson Mandela.

US First Lady Michelle Obama called on Nelson Mandela at his home Tuesday, in a rare visit with the ageing rights icon whose legacy in the fight against apartheid frames her tour of South Africa.

The meeting with the 92-year-old South African Nobel Peace Prize winner was discreetly arranged after Obama and her daughters visited archives chronicling some of his 27 years as a political prisoner.

At the Mandela Foundation, his wife Graca Machel guided Obama, her mother, and her daughters Malia and Sasha walked through an exhibit of his personal photographs and journals, in which he meticulously drafted letters but also kept more mundane notes on his weight and daily routine.

"After leaving here, she proceeded to make a brief courtesy call on former president Mandela," said Achmat Dangor, the head of the foundation, calling the first lady "a lovely woman without any airs".

Their visit added to the symbolism surrounding Obama's journey, with the first black family in the White House meeting South Africa's first black president.

President Barack Obama has called the anti-apartheid struggle his first political cause, and US officials say he has had periodic telephone contact with Mandela, who led the struggle against white-minority rule.

Mandela welcomed the entire Obama  family into his home, including a niece and nephew travelling with them.

Such visits are increasingly rare.

Mandela, who turns 93 next month, has received few guests since he was hospitalised with an acute respiratory infection in January.