In a statement, President Jacob Zuma said Mr Mandela's doctors were "doing everything possible to get his condition to improve".
"He is in good hands," said Mr Zuma.
South Africa's first black president, who is 94, was taken to hospital earlier this month for the third time this year, with a recurrent lung infection.
The statement said Mr Zuma had visited Mr Mandela in hospital in Pretoria on Sunday evening and was told the former president's condition had worsened over the past 24 hours.
"The doctors are doing everything possible to get his condition to improve and are ensuring that Madiba is well-looked after and is comfortable. He is in good hands," said President Zuma, using Mr Mandela's clan name by which he is widely known in South Africa.
