Nelson Mandela's condition has been described as critical for the first time, two weeks after he was admitted to hospital with a lung infection.

His wife Graca Machel has been keeping a constant vigil by the beloved former South African leader's bedside. On June 23 she was joined by other members of his family, including his former wife Winnie, daughter Makaziwe and granddaughter Ndileka.

President Jacob Zuma has also visited, and confirmed that Mr Mandela's health has deteriorated over the past 24 hours.

He said that doctors were still "doing everything possible" to try and improve his situation. But news of the downturn has sparked deep concern among South Africans that the end could be drawing near for their country's first black president.

Outside the Pretoria Heart Hospital where Mr Mandela is being treated, a collection of flowers, cards and balloons has been steadily building up as the nation prays for the iconic anti-apartheid leader, many addressing him as 'Tata' or 'Father'.

"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," Mr Zuma said in his statement, referring to Mr Mandela by his clan name.

On Sunday evening, the 94-year-old's granddaughters Swato Diamini and Zaziwe Diamini-Manaway called on their country to pray for him.

"Let us never forget to pray," they wrote on Twitter. "God lives. He is near. He is real. He is not only aware of us but cares for us."

US President Barack Obama is due to arrive in South Africa with his family on Friday for his first major visit since taking office.

It's a meeting that has been eagerly anticipated for years, between the first black presidents of America and South Africa, but the White House has said that Mr Obama will consult with the Mandela family about whether to visit him.

"Ultimately, we want whatever is in the best interests of his health and the peace of mind of the Mandela family," Ben Rhodes, deputy US national security advisor, said.