JOHANNESBURG: Nelson Mandela's condition was serious when he was admitted to hospital 13 days ago, but the 94-year-old icon's health is improving, South African President Jacob Zuma said on Thursday.

“His condition was serious but he is responding well to treatment and has steadily improved over the last few days,” Zuma told members of the ruling African National Congress at the close of a party conference.

Mandela was airlifted on December 8 from his rural village in the south of the country to a hospital in Pretoria for a series of tests.

Doctors found the former president and hero of the anti-apartheid struggle had a recurrent lung infection and had developed gallstones. He underwent surgery to remove the gallstones on December 15 and is still in hospital.

Zuma sought Thursday to calm jitters over the health of Madiba, as the renowned former statesman is affectionately known by his clan name.

“I have been informed that at his age doctors need to intervene in a very gradual and sensitive way in order to maximise the chances of a full recovery,”said Zuma.

“Madiba is an unparallelled fighter and has always been so. He has met all his health challenges with his tremendous fortitude and grace.”

The Nobel peace laureate has a long history of lung problems dating back decades to when he contracted tuberculosis while in jail as a political prisoner.

He was previously hospitalised for an acute respiratory infection in January 2011, when he was kept as an inpatient for two nights.

Mandela became South Africa's first black president after the country's first all-race elections in 1994 and stepped down after serving one term.