Former South African President Nelson Mandela is recovering from a collapsed lung and could be released from hospital as early as Friday, a source close to Mandela told Reuters on Thursday.
The 92-year-old anti-apartheid icon was admitted to a Johannesburg hospital on Wednesday for what his foundation described as routine tests.
"Mandela was treated by military doctors and should be discharged tomorrow," the source said, asking not to be named.
There has been no official word from the hospital, government or Mandela's foundation on the nature of his illness.
The Nelson Mandela Foundation declined to comment on his health on Thursday but said in an earlier statement: "He is in no danger and is in good spirits."
President Jacob Zuma and the ruling African National Congress appealed for calm on Thursday after the hospitalization set off speculation in local media about Mandela's health.
"President Mandela is comfortable and is well looked after by a good team of medical specialists," Zuma said in a statement. The ANC said there was no cause for alarm.
"He is a 92-year-old and will have ailments associated with his age, and the fact that he stayed the night should not suggest the worst," ANC spokesman Jackson Mthembu said.
According to the University of Maryland Medical Center, a collapsed lung, pneumothorax, is "the collection of air in the space around the lungs," making it more difficult to breathe.
Treatment can vary from allowing the body to repair the problem on its own to placing a tube in the chest to relieve the pressure.
Several members of Mandela's family, including his wife Graca Machel, visited the hospital after his admission, Reuters witnesses said.
Police were called in to control traffic at the Milpark Hospital in a leafy Johannesburg suburb as scores of journalists, photographers and television crews converged on it.
Mandela has not been seen in public since the soccer World Cup final in July last year.
Mandela retired from public life in June 2004 before his 86th birthday, telling his compatriots: "Don't call me, I'll call you."
Since then he has rarely appeared in public and when he did, he appeared increasingly frail. In addition to the World Cup, Mandela appeared at a couple of ANC rallies before general elections in 2009.
Mandela was treated in the 1980s for tuberculosis and later had an operation to repair damage to his eyes. In 2001 he had treatment for prostate cancer.
South African Nobel Peace Prize laureate Desmond Tutu said this week he had met Mandela last week. "He was all right, I mean, he's 92, you know. And he's frail."
Tutu told reporters in Bloemfontein on Thursday: "What more do we want from him? We want him to remain forever, but you know... anything can happen," the SAPA news agency reported.
Mandela -- known by his clan name of Madiba in South Africa -- has been on holiday with his wife, local media reported.