Nelson Mandela has a lung infection but is responding to treatment, the South African government said on Tuesday, as the revered anti-apartheid icon spent his fourth day in hospital.

"Doctors have concluded the tests, and these have revealed a recurrence of a previous lung infection, for which Madiba is receiving appropriate treatment, and he is responding to the treatment," said a statement from President Jacob Zuma's office.

Mandela was previously hospitalised for an acute respiratory infection in January 2011, when he was held for two nights for treatment. He was released in a stable condition for home-based care and intense medical monitoring.