"Doctors are attending to him, ensuring that he has the best possible expert medical treatment and comfort," Xinhua quoted presidential spokesman Mac Maharaj said.
President Jacob Zuma had wished Mandela a speedy recovery, Maharaj said.
"We appeal to the people of South Africa and the world to pray for our beloved Mandela and his family and to keep them in their thoughts.
"We have full confidence in the medical team and know that they will do everything possible to ensure recovery," Maharaj quoted Zuma as saying.
The presidency appealed for understanding and privacy and to give the doctors space to do their work.
Mandela was previously treated for a lung infection and gallstones at Pretoria hospital on Dec 8, 2012. He was discharged on Dec 26 to undergo home-based care at his Houghton home after doctors successfully treated both conditions.
This was Mandela's longest hospital stay since 2001, when he underwent seven weeks of radiotherapy after being diagnosed with prostate cancer.
On March 9, Mandela was admitted to a hospital in Pretoria for a scheduled medical check-up to manage existing age-related conditions. He was discharged a day later.
The presidency said at that time Mandela was "well as before" and his health remained under the management of the medical team.
Mandela served as South African president from 1994 to 1999. He had spent 27 years in prison before elected the country's first black president after the end of apartheid.