Obama 'deeply concerned' about Nelson Mandela's health

"He is as strong physically as he’s been in character and in leadership over so many decades, and hopefully he will, he will come out of this latest challenge," Obama said.

"When we think of a single individual that embodies the kind of leadership qualities that we all aspire to, the first name that comes up is Nelson Mandela," the president continued. "And all of us wish him all the very best.”

Mandela's office said Thursday that the former president was "responding positively" to treatment.

"Doctors are attending to him, ensuring that he has the best possible expert medical treatment and comfort," the South African government said in a statement.

"The doctors advise that former President Nelson Mandela is responding positively to the treatment he is undergoing for a recurring lung infection. He remains under treatment and observation in hospital."

Mandela spent 18 days in the hospital last December as a result of the infection.