JOHANNESBURG (AFP) - Mr Nelson Mandela's wife Graca Machel thanked the world on Monday for its messages of support for the ailing anti-apartheid icon which she said had eased "the burden of anxiety".

South Africa's first black president, 94, was rushed to a Pretoria hospital on June 8 with a recurrent lung infection and remains in a serious condition although he is said to be improving.

"Our gratitude is difficult to express. But the love and peace we feel give yet more life to the simple 'Thank you!'," Mrs Machel, 67, said in a message.

"So much love and generosity from South Africans, Africans across the continent, and thousands more from across the world, have come our way to lighten the burden of anxiety; bringing us love, comfort and hope," she said.

"The messages have come by letter, by SMS, by phone, by Twitter, by Facebook, by email, cards, flowers and the human voice, in particular the voices of children in schools or singing outside our home," Mrs Machel wrote.

"We have felt the closeness of the world and the deepest meaning of strength and peace." She referred to Mr Mandela by the clan name Madiba by which he is affectionately known in South Africa.

"Madiba once said: 'What counts in life is not the mere fact that we have lived. It is what difference we have made in the lives of others.' I have thought of his words on each occasion the world stood with him, making a difference to him, in his healing."

Mr Mandela, who is due to celebrate his 95th birthday on July 18, has been hospitalised four times since December.

South African President Jacob Zuma said on Sunday that Mr Mandela was showing a "sustained" improvement after more than a week in hospital although his condition remained serious.

The Nobel peace laureate had appeared frail and distant in the last images broadcast of him at the end of April during a visit by Mr Zuma and other ANC leaders.

Mrs Graca Machel, who became Mr Mandela's third wife in 1998, has been at her husband's bedside since the first day of his latest hospitalisation.

Unlike Winnie Madizikela-Mandela, Mr Mandela's second wife and the anti-apartheid struggle's former muse who has been seen many times at the hospital entrance, nobody has seen Mrs Machel in the last few days.

Journalists deployed outside Pretoria's Mediclinic Heart Hospital suspect that Mrs Machel is sleeping inside.

Mrs Machel was the wife of another head of state, president Samora Machel, the hero of Mozambique independence who died in 1986.

Although he has retired from public life, Mr Mandela is still venerated by an entire people who see him as the incarnation of the end of three centuries of white-minority rule in South Africa.

Black South Africans were able to vote for the first time in 1994.

Mr Mandela is admired throughout the world for his lifelong sacrifice in fighting the brutal regime of racial segregation installed with apartheid in 1948, and for his role in bringing multiracial democracy to South Africa, a country many feared would disintegrate into civil war.