WASHINGTON: President Barack Obama heads to Africa on a long awaited first major tour Wednesday, but a trip meant to make up for lost time is instead being overshadowed by Nelson Mandela's fading health.

Obama will leave Andrews Air Force base outside Washington for a weeklong trip that is scheduled to include stops in Senegal, South Africa and Tanzania.

Yet Mandela's fragile state of health has sparked speculation that the tour could be halted, or radically changed, if the anti-apartheid icon passes away in the hours before Obama leaves, or while he is on the continent.

The White House has said that it will defer to Mandela's family on whether the president would visit his ailing 94-year-old political hero in the Pretoria hospital where he has been for nearly three weeks.

South Africa's foreign minister Maite Nkoane Mashebane said that while Obama would have loved to see Mandela, a meeting with the former South African leader would be impossible as he was "indisposed."

The men met 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 on the telephone.

But there have been no face-to-face meetings between the first black presidents of the United States and South Africa since Obama was elected in 2008.

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 for a brief stop in Ghana.