Mr Mandela was admitted to hospital 12 days ago and has been treated for a lung infection and gallstones.
Doctors are satisfied with his progress but say he needs “extraordinary care” because of his age.
Mr Mandela led the struggle against white-minority rule before being elected the first black president in 1994.
He contracted tuberculosis while on the windswept Robben Island where he served 18 of the 27 years he was imprisoned for sabotage.
His lungs are said to have been damaged when he worked in a prison quarry.
Nelson Mandela remains an important political symbol in South Africa, says the BBC’s Karen Allen, in Johannesburg.
More so at the moment, our correspondent adds, as the governing ANC has re-selected Jacob Zuma as party President and returned Cyril Ramaphosa – the millionaire businessman – back into the political spotlight, as his deputy.
Once a leading trade unionist, Mr Ramaphosa became the symbol of black capitalism in South Africa after the African National Congress (ANC) came to power at the end of white minority rule in 1994.
When Mr Mandela overlooked him for the role of his deputy, he was said to have been so upset that he refused to attend Mr Mandela’s inauguration as president.
Source: BBC
Nelson Mandela recovering but stays in hospital
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The former South African president, Nelson Mandela, is being kept in hospital despite “looking much better”, according to the government. Presidential spokesman Mac Maharaj said doctors treating the 94-year-old are “in no hurry to send him home”.