"Doctors are attending to him, ensuring that he has the best possible expert medical treatment and comfort," the government's statement said. "We appeal to the people of South Africa and the world to pray for our beloved Madiba [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," South African President Jacob Zuma said in the statement.

Mandela, 94, spent 18 days in the hospital in December for a lung infection and gallstones.

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The anti-Apartheid activist also was admitted to a Pretoria hospital on March 10 for one-day of routine medical tests and to "manage existing conditions in line with his age," a spokesman for the South African president's office said, according to ABC News.

Mandela has been admitted to the hospital four times in just over two years. He first was diagnosed with tuberculosis in the 1980s while detained on the treacherous Robben Island, where he served 18 of his 27 years of imprisonment for sabotage.

Mandela's hard labor in the prison quarry was said to have heavily damaged his lungs, according to the BBC.

The treatment he received in December 2012 was his longest hospital stay since leaving prison in 1990.

Mandela, who is credited with changing race relations in South Africa, remains hugely popular in the country. He became a global statesman after leaving office in 1999, mediating conflicts in some of the world's most troubled spots.

Mandela has since retired to his childhood home in Qunu.

The government occasionally releases photos of the former president after high-profile visitors meet with him in Qunu, including a recent meeting with former Secretary of State Hillary Clinton and family.

The Associated Press contributed to this report.