The 94-year-old, who served as South African president from 1994 to 1999, was hospitalised Wednesday night. He spent 27 years in prison before being elected the country's first black president after the end of apartheid.
"He remains under treatment and observation in hospital," Xinhua quoted the presidential spokesperson, Mac Maharaj, as saying, late Thursday.
"The presidency thanks the media and the public for their cooperation in respecting the privacy of Madiba and his family," he added.
Earlier in the day, the presidency said, "Doctors are attending to him, ensuring that he has the best possible expert medical treatment and comfort."
South African President Jacob Zuma had wished Mandela a speedy recovery.
"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.
Mandela was previously treated for a lung infection and gallstones at Pretoria hospital Dec 8, 2012. He was discharged Dec 26 to undergo home-based care at his Houghton home.
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, one of the country's three capital cities, 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.