Nelson Mandela in critical condition

The condition of former President Nelson Mandela is critical, the South African presidency has said, two weeks after the 94-year-old anti-apartheid icon was admitted to a hospital with a recurrent lung infection.

President Jacob Zuma and African National Congress Deputy President Cyril Ramaphosa visited Mandela on Sunday evening, Mac Maharaj, presidential spokesman, said in a statement.

The condition of the revered world leader, who was admitted to the Mediclinic Heart Hospital in Pretoria on June 8, has worsened in the past 24 hours.

"They were briefed by the medical team who informed them that the former president's condition had become critical over the past 24 hours," Maharaj said.

"The doctors are doing everything possible to get his condition to improve and are ensuring that Madiba is well-looked after and is comfortable. He is in good hands," Zuma was quoted as saying in the statement.

They also discussed his condition with Mandela's wife Graca Machel, who is believed to have been staying in an adjacent room at the hospital.

The latest statement from the Presidency is the strongest indication yet that all is not well with the ailing elder statesman, who will turn 95 in three weeks' time.

The news came amid mounting public outcry after it was learnt that the ambulance transporting Mandela to the hospital from his home in Johannesburg in the early hours of June 8 broke down and paramedics had to treat him for almost forty minutes before a second one arrived.

There was also concern about how Mandela had been transferred to the second ambulance in the bitterly cold winter morning while he was suffering from a lung disorder.

The opposition Democratic Alliance has called for a full enquiry into the incident which was disclosed by an American news network on Friday and confirmed by the Presidency, but kept away from the South African public and media.

"All care was taken to ensure that Madiba's medical condition was not compromised by the unforeseen incident. Doctors attending to Madiba are satisfied that the former president suffered no harm during this period," said Maharaj while responding to the report.

Maharaj declined to comment on the CBS News report that Mandela's liver and kidneys were functioning at 50 per cent and that Mandela had not opened his eyes in days and was unresponsive.

"Our reports are based on the reports we receive from doctors. We avoid clinical details because we want to ensure no transgression into the privacy of Mandela and his family is ensured," Maharaj said.

"The presidency needs to ensure the dignity of the former president and need to ensure the circumstances are not surrounded by undignified speculative reports," he added.

This is the revered world leader's fourth hospitalisation since December.

Mandela has 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.

In December last year, 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.

In March, he was admitted for an overnight scheduled check-up before returning to the hospital that month for 10 days.

Mandela, who turns 95 in July, has not been seen in public since the World Cup final in South Africa in July 2010.

Mandela, one of the world's tallest statesmen, led the movement to replace the apartheid regime of South Africa with a multi-racial democracy.

He served as South Africa's first black president between 1994 and 1999. He won the Nobel Peace Prize in 1993.