"Doctors advise that due to the lung infection former President Mandela had developed a pleural effusion (excess fluid in the lungs) which was tapped," Presidential spokesperson Mac Maharaj said in a statement today.
"This has resulted in him now being able to breathe without difficulty. He continues to respond to treatment and is comfortable, the statement said.
"Former President Nelson Mandela was admitted to hospital during the late hours of 27 March due to a recurrence of pneumonia," it said.
After he was admitted to the hospital, President Zuma had said Madiba (Mandela is often fondly known by his clan name) was doing "very well" so far and asked people not to "panic".
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The President office had said yesterday that Mandela was in "good spirits" and making "steady progress".
"The Presidency wishes to advise that former President Nelson Mandela is in good spirits and enjoyed a full breakfast this morning," the statement had read.
Earlier this month, Mandela spent a night at a Pretoria hospital where he underwent a successful medical examination.
Three months ago, he was admitted for 18 days for treatment of the lung infection and surgery to extract gallstones. It was his longest stint in hospital since his release from prison in 1990.
Mandela had a long history of lung problems, dating back to the time when he was a political prisoner on Robben Island during apartheid. He contracted tuberculosis in 1988 during his 27 years in prison.