Pretoria - US President Barack Obama has landed in South Africa, the second stop in his three-country tour of Africa.
Mr Obama said earlier he did not expect to see former President Nelson Mandela, who is critically ill in hospital.
Leaving Senegal, he told reporters on board Air Force One: "I don't need a photo op."
Meanwhile, Mr Mandela's ex-wife, Winnie Madikizela-Mandela said the former leader had made "a great improvement" in recent days, but was "still unwell".
Correspondents say security is tight in the streets near Mediclinic Heart Hospital in the capital, Pretoria, where the 94-year old is being treated for a lung infection.
Ministers, politicians, Mr Mandela's physician and family members were among those visiting the ex-leader on Friday, his 21st day in hospital.
'Hero for the world'
President Obama's plane landed at a military airbase near Pretoria on Friday evening. He has meetings scheduled in the capital on Saturday morning.
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At the scene
BBC News, Pretoria
Outside the hospital where messages of goodwill are swelling by the day sit two paintings - one of Nelson Mandela, the other of Barack Obama.
The symbolism couldn't be more potent, as America's first black president arrives on the second leg of his Africa tour.
A brief protest outside the hospital suggested not everyone welcomes his visit - some are angry at what they see as the "militarisation" of US foreign policy - but the mood surrounding the president's tour is overwhelmingly positive.
People continue to pin messages of support on the railings outside the hospital.
One of the most touching tributes reads: "Today, we have better schools, I receive grant [sic] all thanks to you. Nutrition donates food to our creche because of you. If it wasn't for you, what would South Africa be?"
But Mr Obama said earlier he did not expect to see the ailing ex-leader during his visit to South Africa.
"I don't need a photo op," he said aboard Air Force One after leaving Senegal. "The last thing I want to do is to be in any way obtrusive at a time when the family is concerned with Nelson Mandela's condition."
He went on to say: "I think the main message we'll want to deliver is not directly to him, but to his family - is simply profound gratitude for his leadership all these years, and that the thoughts and prayers of the American people are with him, his family and his country."
Mr Obama met Mr Mandela in 2005 when he was still a US senator. Both men became the first black presidents of their nations and have received the Nobel Peace Prize.
The US president has described Mr Mandela as a "hero for the world", whose "legacy will linger on through the ages", and who had inspired his own activism as a student.
'Hero for the world'
Earlier on Friday, Mrs Madikizela-Mandela told reporters outside Mr Mandela's former home in Soweto: "I'm not a doctor but I can say that from what he was a few days ago there is great improvement, but clinically he is still unwell."
She also thanked domestic and international media for their coverage, but added that some of the reports had caused the family distress.
"We had no idea of the love for us out there," she said.
"There may be problems here and there when some of you get carried away and talk about our father in the past tense," she said, adding:
"If sometimes we sound bitter, it is because we are dealing with a difficult situation."
Earlier this week, Mr Mandela's eldest daughter criticised the international media camped outside the Mediclinic Heart Hospital, calling them "vultures".
Meanwhile there are mixed emotions among the many well-wishers gathering there, the BBC's Karen Allen reports from the scene. People are anxious about his health but also want to express their pride in the man many consider the father of the nation.
"There is no sadness here. There is celebration. He is a giant," 57-year-old Nomhlahla Donry, whose husband served time with Mr Mandela, told the AFP news agency.
"We are saying let him live long because we like him and he has done a lot for us in this world. He's done a lot of amazing things and we really love Tata," local resident Alfred Makhathini told Reuters.
South Africa's ruling African National Congress (ANC) said it would hold vigils each day that the former leader remained in hospital, and the distinctive black, green and gold colours of the party are much in evidence.
But the party denied it was exploiting the occasion to canvas for votes ahead of next year's elections.
"We love our ANC regalia and we have every right to wish Madiba well," party spokesman Jackson Mthembu said.
Meanwhile, a court in the Eastern Cape has granted an application brought by the Mandela family to exhume three of his children and two other relatives and rebury them in the family cemetery in Qunu, which is where the former leader wants to be buried, their lawyers say.
The BBC's Pumza Fihlani in Johannesburg says this is one of the reasons why the family held a meeting earlier this week.