President Jacob Zuma urged his countrymen and the world to pray for the beloved leader and his family.
The health of the ailing 94-year-old icon has been a concern in recent years. He was admitted to a hospital in Pretoria for a brief period earlier this month.
Mandela's close friend George Bizos, an advocate, said that there was no big problem with the former although his memory wasn't what it used to be earlier, a news agency reported.
Mandela had undergone a gallstone surgery in December and the doctors also attended his lung infection. He was discharged on December 26 after a stay of over two weeks. It was his longest stay in hospital since 2001 when he had undergone seven weeks of radiotherapy after being diagnosed with prostate cancer.
Mandela, a iconic leader in South Africa's anti-Apartheid struggle, served as its president from 1994-99. He had spent 27 years in prison before being elected the first black president of South Africa in the post-Apartheid era.