JOHANNESBURG (Reuters) - 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. President Jacob Zuma and the ruling African National Congress appealed for calm on Thursday after the hospitalisation set off speculation in local media about Mandelas 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 reason to panic. 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. Mandela has not been seen in public since the soccer World Cup final in July last year.