President Barack Obama has said his country's thoughts are with Nelson Mandela, and hailed the Nobel peace laureate "an inspiration to the world".
The US leader was speaking during an address in Pretoria, South Africa, following talks with President Jacob Zuma.
During his speech, Mr Obama said: “The struggle here against apartheid [and] for freedom, Madiba's (Mr Mandela's) moral courage, this country's historic transition to a free and democratic nation, has been a personal inspiration to me.
"It has been an inspiration to the world, and it continues to be in so many regions that are divided by conflict, sectarian disputes, religious or ethnic wars, to see what happened in South Africa. The power of principle and people standing up for what's right I think continues to shine as a beacon.”
Mr Obama added: “The outpouring of love we have seen in recent days shows that the triumph of Nelson Mandela and this nation speaks to something very deep in the human spirit.
”The yearning for justice and dignity that transcends boundaries of race and class and faith and country, that's what Nelson Mandela represents, that's what South Africa at its best can represent to the world.“
A White House statement released earlier in the week said he and his wife Michelle would offer their thoughts and prayers at the family's difficult time during their week long trip to South Africa.
It read: "Out of deference to Nelson Mandela's peace and comfort and the family's wishes, they will not be visiting the hospital."
On Friday Obama said: "I don't need a photo op. The last thing I want to do is to be in any way obtrusive at a time when the family is concerned about Nelson Mandela's condition."
A photo of his 2005 meeting with Mr Mandela hangs in his personal office at the White House - their only meeting, when Mr Obama was a senator.
The 94-year-old national icon has been in hospital for three weeks with long-running lung problems.
The US president has said the imprisoned activist's willingness to risk his life for the cause of equal rights helped inspire his own political activism. He said his message during the visit will draw on the lessons of Mr Mandela's life, with a message that "Africa's rise will continue" if its people are unified instead of divided by tribe, race or religion.
"I think the main message we'll want to deliver, if not directly to him but to his family, is simply a profound gratitude for his leadership all these years and that the thoughts and prayers of the American people are with him and his family and his country," Mr Obama said on his flight into the country.
The US leader will also pay tribute to the fight against apartheid by visiting the Soweto area and meeting students at the University of Johannesburg.
At least 176 young people were killed in Soweto township 27 years ago this month during a youth protest against the apartheid regime's ban against teaching local Bantu languages. The Soweto Uprising catalysed international support against apartheid, and June is now recognised as Youth Month in South Africa.
Additional reporting by agencies