Obama: My thoughts are with Madiba
Obama also confirmed on Thursday that he still planned to travel to South Africa in the coming days. He said that if Madiba died, his legacy would live on.
The US president was due to fly into South Africa on Friday. He is currently in Senegal, the first leg of his trip to Africa.
Obama's South African schedule does not include a visit to Mandela.
Obama will take part in bilateral talks, preside over a Young African Leaders initiative meeting and visit Robben Island, where Mandela was imprisoned, according to an outline of his planned activities released by the international relations and co-operation department on Tuesday.
He arrives in Johannesburg on Friday and will visit Soweto on Saturday.
He will also visit various sites in Cape Town, and deliver an address at the University of Cape Town.
Meanwhile, Mandela's fellow Robben Islander Laloo Isu Chiba told the Mail & Guardian on Thursday: "All we can do is express the hope of the nation and say that he's in our prayers," said Chiba, who spent 18 years on Robben Island from 1964 to 1982, helped transcribe Mandela's biography Long Walk to Freedom by condensing Mandela's handwriting to a much smaller script before it was smuggled out into the world by Mac Maharaj (now presidential spokesperson).
"People want to be preparing in the event that he dies, I am very saddened that this is taking place. As the Ahmed Kathrada Foundation, we'd rather not comment at this point, we will deal with it as it comes … "
Chiba joined the MK in 1961 but was soon in the second national high command under Wilton Mkwayi.
Detained under the 90-day detention law, Chiba was severely beaten and tortured but did not implicate his comrades. In July 1964 Chiba was arrested alongside Babla Saloojee, Paul Joseph, Maharaj, Steve Naidoo and Amien Kajee.
In October of that year, he was charged with sabotage and membership of a banned organisation and sentenced to 18 years in prison.
President Jacob Zuma has cancelled a trip to neighbouring Mozambique, intensifying speculation about a deterioration in the health of Mandela.
Zuma made his decision not to leave the country after visiting the 94-year-old former president late on Wednesday in the Pretoria hospital where he has been receiving treatment for a lung infection for nearly three weeks.
"Clearly the issue of seriousness has been such that President Jacob Zuma has cancelled his trip," Maharaj told Talk Radio 702.
He declined to comment on reports that Mandela was on life support, saying: "I cannot confirm any clinical details."
Reports by CNN and the Citizen on Wednesday said Mandela was on life support, and Maharaj told the M&G late on Wednesday night that commenting on the matter would break doctor/patient confidentiality.
The Citizen also reported that Madiba was experiencing kidney failure.
The deterioration in his health at the weekend to "critical" from "serious but stable" caused a perceptible switch in the national mood, from prayers for his recovery to preparations for a fond farewell. He had been admitted to the Pretoria Heart Mediclinic on June 8 for a recurring lung infection.