"We have seen the latest reports from the South African government that former president Mandela is in critical condition," National Security Council spokeswoman Caitlin Hayden said.
"Our thoughts and prayers are with him, his family and the people of South Africa."
Obama leaves Wednesday on a tour of Africa that will take him to Senegal, South Africa and Tanzania, but he has said he will defer to Mandela's family regarding any meeting with the anti-apartheid icon.
The possibility of a meeting between the first black presidents of both South Africa and the United States has been hotly anticipated for years.
But the declining health of Mandela, 94, who has been fighting a lung infection, and Obama's failure to visit South Africa until his second term, have left the prospect in doubt.
Obama will stay overnight in Johannesburg and Cape Town during his trip, and plans to visit Robben Island, where Mandela spent 18 of his 27 years in prison.
Obama met Mandela soon after he was elected a US senator in 2005, when the former South African president was visiting Washington, and the two have spoken several times since on the telephone.
Obama's wife, Michelle, was able to visit to Mandela during her trip to Africa two years ago, and she said it was the most moving moment of her visit.
Obama will leave Washington on Wednesday for the three-nation tour meant to emphasize economic potential and democratic development.
He will not, however, be visiting Kenya, which was the homeland of his late father and where he still has living relatives.
The fact that Kenya's recently elected President Uhuru Kenyatta is facing trial at the Hague for allegedly orchestrating deadly violence in 2007 and 2008 following previous polls had stymied plans for a possible visit.
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