President Jacob Zuma said Nelson Mandela is "responding very well to the treatment" as he urged the country not to panic.
He told BBC News: "Of course I have been saying to people, you should bear in mind Madiba is no longer that young and if he goes for check-ups every now and again, I don't think people must be alarmed about it. I would like to really say the country must not panic."
More top news
Pitt was due to attend the premiere of a new documentary he narrated, but instead will focus on his "family situation".
ITV News spoke to six Democrats and Republicans to get their views on the dramatic showdown between the two presidential hopefuls.
Nicolas Sarkozy would give Britain the chance to reverse the Brexit vote if he is elected president of France.