US President Barack Obama said Thursday he was "deeply concerned" for the health of Nelson Mandela, after the former South African president was again admitted to hospital.

"We are deeply concerned with Nelson Mandela's health -- he is a hero, I think, to all of us," Obama said as he met four leaders from sub-Saharan Africa at the White House.

"We will be keeping him in our thoughts and prayers, and his entire family," Obama said.

"He is as strong physically as he has been in character and in leadership over so many decades. Hopefully he will come out of this latest challenge."

"We all recognize that he has given everything to his people, the people of South Africa, the people of the continent and he has ended up being an inspiration to all of us," Obama said.

"When we think of a single individual that embodies the kind of leadership qualities that I think we all aspire to, the person's name that comes up is Nelson Mandela. So we wish him all the very best."

Mandela, 94, was readmitted to hospital with a lung infection, the latest health scare for the much-loved anti-apartheid icon.

Obama made the remarks at the White House after meeting President Ernest Koroma of Sierra Leone, President Macky Sall of Senegal, President Joyce Banda of Malawi and Prime Minister Jose Maria Neves of Cape Verde.