South Africa's ailing anti-apartheid hero and former President Nelson Mandela remained in a "critical but stable" condition after nearly four weeks in hospital, the government said today.

Mandela is receiving treatment for a recurring lung infection, his fourth hospitalisation in six months. The latest health update from the government followed a visit to the hospital by current President Jacob Zuma.

Mandela's wife Graca Machel, speaking during an event at the Nelson Mandela Foundation, said his hospitalisation had offered South Africans from all walks of life a chance once again to be united.

"Although Madiba sometimes may be uncomfortable, very few times he is in pain, but he is fine," Machel said, using the clan name by which Mandela is affectionately known.

The failing health of 94-year-old Mandela, a figure admired globally as a symbol of struggle against injustice and racism, has reinforced a realisation that the father of the post-apartheid South Africa will not be around for ever.

While Mandela lies in hospital, a row over grave sites has split his family.

Two years ago, Mandela's grandson moved the bodies of three of Mandela's children from a family cemetery in Qunu, the village where Mandela spent most of his childhood, to the nearby village of Mvezo.

A High Court yesterday ordered that the remains be exhumed and reburied in Qunu - an edict that was carried out on Wednesday night.

In a court affidavit filed last week, Mandela's eldest daughter, Makaziwe, argued for an urgent hearing, saying her father was in a "perilous" condition and breathing with the aid of life-support.