U.S. first lady Michelle Obama on Tuesday paid a previously unannounced visit to former South African president Nelson Mandela at the beginning of a tour of southern Africa.

Mrs. Obama met with the 92-year-old anti-apartheid icon at his home in Johannesburg. The wife of U.S. President Barack Obama was accompanied by the couple's two daughters, Malia and Sasha, and by her mother Marian Robinson.

A photograph released by the Nelson Mandela Foundation showed Mr. Mandela sitting on a couch next to Mrs. Obama during the 20-minute visit.

Mr. Mandela has been restricted to his home since he was hospitalized with an acute respiratory infection in January. He rarely receives visitors.

The first lady is on a week-long trip to Africa where she will promote youth empowerment, encouraging young Africans to take a leading role in helping solve the continent's problems that include poverty, disease and climate change.

Mrs. Obama, who arrived in Pretoria on Monday, will spend most of Wednesday in the Johannesburg suburb of Soweto. Her trip includes visits to Cape Town and Robben Island where Mr. Mandela was held for most of the 27 years he was imprisoned for leading the struggle against apartheid, a South African government policy of racial separation.

Mr. Mandela served one term as South Africa's president before stepping down in 1999. He retired from public life in 2004.

After visiting South Africa, Mrs. Obama's will spend two days in neighboring Botswana, arriving there on Saturday.