Nelson Mandela's former wife, Winnie Madikizela-Mandela, has visited the Pretoria hospital where he is being treated for a lung infection.

Mr Mandela's second wife was accompanied by their daughter Zindzi Mandela-Hlongwan at the Mediclinic Heart Hospital.

Their eldest daughter, Zenani Dlamini, who is South Africa's ambassador to Argentina, has also returned to South Africa.

Mr Mandela's condition remains serious but stable, the South African government said.

He was admitted to a clinic early on Saturday night as his condition worsened.

The former president has been particularly vulnerable to respiratory problems since contracting tuberculosis during his 27-year imprisonment under apartheid.

In a statement President Jacob Zuma repeated his call for the country to pray for the ailing 94-year-old anti-apartheid leader.

Mr Mandela was admitted to a Pretoria hospital early on Saturday after his condition deteriorated. He has been hospitalised several times this year.

In early March, he was admitted to a Pretoria hospital for a scheduled check-up and was discharged the following day.

Later the same month, Mr Mandela was admitted again to a hospital for the same ailment and was discharged nine days later.

Mr Mandela served as South African president from 1994 to 1999. He spent 27 years in prison before he was elected the first black president in the country after the end of apartheid.