President Jacob Zuma issued a brief statement Saturday in which he asked that Mandela's privacy be respected. He did not say at which hospital Mandela was being treated.
Mac Maharaj, Zuma's spokesman, said he could not elaborate but that he would be issuing regular updates.
Mandela became South Africa's first black president in 1994 after spending 27 years in prison for his fight against racist apartheid rule, and was awarded a Nobel Peace Prize for his efforts.
His public appearances have become increasingly rare, and he was hospitalized last year for a few days with an acute respiratory infection.
