Nelson Mandela
Barack Obama 'Deeply Concerned' for Nelson Mandela's Health - Reuters

Nelson Mandela, the first black president of South Africa, who was admitted to hospital following recurrence of a lung infection, is "responding positively" to treatment, said an official statement.

"He remains under treatment and observation in hospital. Doctors are attending to him, ensuring that he has the best possible expert medical treatment and comfort," the statement from the South African government said.

President Jacob Zuma, who urged the people to pray for their "beloved Madiba", told the BBC that Mandela is doing "very well" and South Africans "must not panic."

"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," said Zuma.

His statement came as South Africans were increasingly concerned over the health of the anti-apartheid icon.

"I'm so sorry. I'm sad. I just try to pray all the time. He must come very strong again," a Johannesburg resident, Obed Mokwana told the Associated Press.

Another resident named Samson Ndlovu told Reuters: "He will come out fine, let's just have faith in god."

World leaders have also expressed their concern. US President Barack Obama said: "We are deeply concerned with Nelson Mandela's health - he is a hero, I think, to all of us. We will be keeping him in our thoughts and prayers, and his entire family," said.

Hoping that Mandela will come out of "this latest challenge", Obama said: "He is as strong physically as he has been in character and in leadership over so many decades."

In December last year, the 94-year-old had spent nearly three weeks in hospital for lung infection and gallstones. He has become increasingly frail in recent years and has been admitted to hospital several times.