U.S. President Obama heads a soccer ball at Ubungo Power Plant in Dar es Salaam

U.S. President Obama heads a soccer ball at Ubungo Power Plant in Dar es Salaam (Jason Reed, Reuters)

But Obama said Kenya was close to his heart and renewed a promise to return there while in office. Obama and his family returned to Washington on Tuesday evening.

In the meantime, the continent will be watching to see how he follows up on the pledges made during this visit.

If Mandela dies, Obama will almost certainly return for the funeral. A picture of the two men together hangs in the family's residence at the White House, next to a photograph of Mandela with first lady Michelle Obama and the two Obama daughters, taken when they went to South Africa two years ago.

Obama has invited African leaders for a summit next year in Washington and promised to send high-level Cabinet secretaries on trips to the continent.

"Only time will tell whether this trip signifies the start of a more active chapter in U.S.-Africa relations," Downie said. "It's easy enough to make nice speeches, but it's the day-to-day policy substance that matters."

(Editing by Pascal Fletcher and Kevin Liffey)