President Zuma has asked for prayers for Mandela and family.
South African presidential spokesman, Mac Maharaj, said that the 94-year-old Nobel Peace Laureate was conscious when he was admitted just before midnight.
“The doctors would prefer to work on the side of caution rather than take any risks,” said Mr. Maharaj, who served prison time with Mandela during the 1970s for their roles in the struggle against Apartheid.
Mr. Mandela was discharged from a Pretoria hospital on December 26 after two weeks of treatment for a lung infection and surgery to remove a gallstone. It was his longest hospitalisation since he was released from prison in 1990.
The anti-Apartheid icon has been receiving care during the past three months at his home in a Johannesburg suburb.
“We appeal to the people of South Africa and the world to pray for our beloved Madiba and his family and to keep them in their thoughts,” said President Jacob Zuma, using Mr. Mandela’s clan name.
“We have full confidence in the medical team and know that they will do everything possible to ensure his recovery,” Mr. Zuma added.
In January 2011, Mr. Mandela was admitted to hospital for several days with an acute respiratory infection, sparking concern for his wellbeing.
Mr. Mandela spent 27 years in jail for playing a leading role in the struggle against white minority rule and became South Africa’s first black president in 1994, following the first all-race elections. He served one five-year term before stepping down.
He is admired for seeking reconciliation among South Africans and won the Nobel Peace Prize in 1993.
In the 1980s, while still incarcerated by the Apartheid regime, Mr. Mandela suffered from tuberculosis and his lungs were said to have been damaged during years of hard labour in prison.
Since retiring from public life, Mr. Mandela has in recent years tried to spend as much time as possible in Qunu, his rural ancestral village in the remote Eastern Cape Province. However, since his hospitalisation in 2012, Doctors preferred that he stay in Johannesburg, close to quality medical care.