Report: Mandela placed on life support

"We continue to pray for him and his family, and obviously all of the South African people to whom he means so much as well as his admirers around the world," White House press secretary Jay Carney told reporters aboard Air Force One. "And at this time, we just don't have any new information. And we simply hope that he recovers."

In a statement, the South African government said Mandela's doctors "continue to do their best to ensure his recovery, well-being and comfort."

While in South Africa, the president is scheduled to give a speech on Africa policy at the University of Cape Town. The site has particular resonance as the place that the late Sen. Robert F. Kennedy gave his famous 1966 speech drawing parallels between apartheid in South Africa and Jim Crow in the United States.

Obama had also hoped to visit with Mandela, as well as travel to Robben Island, the prison where the former president and other political prisoners were held.

Those plans — and the remainder of the president's trip, which includes stops in Senegal and Tanzania — could change quickly if Mandela were to pass away.