South Africa's government confirmed early on Friday that former president Nelson Mandela was undergoing specialised tests in hospital but that his health was not in danger.
"Medically there is no need to panic. Dr Mandela suffers from ailment common to people of his age, and conditions that have developed over years," said Vice President Kgalema Motlanthe in a statement.
"We may recall that he has suffered from tuberculosis whilst on Robben Island and has had previous respiratory infections."
Motlanthe and South Africa's surgeon-general will brief the media at midday on the 92-year-old's health which has drawn increasing concern from the public.
"I want to assure the nation and the world that the former president is in high spirit, and has been visited by his family and friends," said Motlanthe in a statement issued around midnight Friday (2200 GMT).
The country's first democratic leader, who spent the bulk of his 27 apartheid jail term on Robben Island, was undergoing specialised tests and investigations, he said.
A source close to Mandela told AFP on Thursday night that Mandela was "very sick" but that his condition was "not life threatening".
"He came in for a check-up but the doctor decided to keep him in for observation. He is still not well but we expect him to be released tomorrow."
No hospital or medical official contacted by AFP would confirm that Mandela would soon be discharged from Milpark hospital in Johannesburg where he was admitted on Wednesday.
Mandela was suffering from a respiratory condition, believed to be bronchitis, and put on a ventilator in the early hours of Thursday after he had difficulty breathing and speaking, The Times newspaper reported on Friday.
On Wednesday, his office said he was undergoing "routine" tests but that his health was not in jeopardy.
Mandela was imprisoned for his role in the fight against apartheid in South Africa, emerging in 1990 to lead the transition to democracy.