Ailing Mandela casts shadow over visit to Africa by Obama
US president's Africa tour plunged into doubt as anti-apartheid hero clings to life in hospital
Barack Obama landed in Senegal to begin his first major tour of Africa as concern grew over the health of Nelson Mandela.
The possibility that critically ill Mandela could fade away within days has sparked uncertainty about Obama's itinerary.
Mandela remained in critical condition in hospital despite tentative signs of improvement, as Obama led a chorus of support for the "hero for the world".
South African President Jacob Zuma reported that he "remains critical but is now stable."
"He is much better today," Zuma said. "The medical team continues to do a sterling job. We must pray for Tata's (father's) health and wish him well."
The US president's plans to visit Senegal, South Africa and Tanzania over the next week could be complicated, shifting the focus of a trip meant to ease the disappointment of Africans who believe Obama's presidency has fallen short of expectations.
The White House has refused to say exactly what contingency plans are in place for the week-long trip, designed to highlight Africa's emerging economic potential and also to promote youth and health programmes.
But Ben Rhodes, a deputy national security adviser, said it would be left to Mandela's family to decide whether Obama could visit his ailing 94-year-old political hero in the Pretoria hospital where he has been for nearly three weeks.
"He is a personal hero," Obama said in Senegal, among the many African countries that have benefited from Mandela's example of a peaceful transition to power.
"I think he is a hero for the world, and if and when he passes from this place, one thing I think we all know is that his legacy is one that will linger on throughout the ages."
Obama met Mandela in 2005, when the former South African president was in Washington and Obama was a newly elected senator, and the two have spoken several times since by telephone.
But there has been no face-to-face meeting between the first black presidents of the United States and South Africa since Obama was elected in 2008.
Obama stepped off the plane with First Lady Michelle followed by his daughters Malia and Sasha. He was greeted by Senegal President Macky Sall and First Lady Marieme Sall, as well as US Ambassador to Senegal Lewis Lukens and members of President Sall's cabinet.
The White House sees Obama's visit as a chance to make up for lost time, as the president was unable to fit in a visit to sub-Saharan Africa in his first term, apart from a brief stop in Ghana.
There has also been disappointment on the continent, after Obama's 2008 election caused euphoria and an expectation that the son of a Kenyan would put Africa policy at the top of his agenda.
And US officials are aware that emerging economic opportunities and energy resources in Africa have attracted a clutch of interest from rising rivals.
Washington noticed that President Xi Jinping professed a "sincere friendship" with Africa when he visited the continent on his first foreign tour in March as part of an economic and diplomatic offensive.