Former South African president Nelson Mandela remains in a critical condition in hospital.
"Doctors are doing everything possible to ensure his wellbeing and comfort," President Jaco Zuma told a news conference in Johannesburg.
He declined to answer specific questions about Mr Mandela's condition, saying he had no further information.
Mr Mandela is being treated for a recurrence of a lung infection.
Last night was the first time the word critical was used by the South African government in describing his health.
Mr Zuma broke the news after visiting Mr Mandela in hospital yesterday evening.
He was told by doctors "that the former president's condition had become critical over the past 24 hours".
Mr Zuma said the 94-year-old anti-apartheid icon was "in good hands".
Mr Zuma last night met Graca Machel, Mr Mandela's wife, at the hospital in Pretoria and discussed the former leader's condition.
Presidential spokesman Mac Maharaj said the Mandela family is going through a stressful time.
He said: "We want to assure the public that doctors are working away to try and get his condition to improve. Let us keep our fingers crossed.
"Let's hope that he is able to improve, but at the same time let us appreciate that it is critical."
Mr Mandela has suffered repeated bouts of illness in recent months and has been admitted to hospital four times since December.
He has been in intensive care since he was last admitted to hospital on 8 June.
Mr Mandela's daughter Makaziwe said the family was taking each day as it came and enjoying as much time as possible with a man who, to them, is simply a father, grandfather or great-grandfather.
"He is at peace with himself," she told CNN. "He has given so much to the world. I believe he is at peace."
Since stepping down in 1999 after one term as president, Mr Mandela has stayed out of active politics.
His last public appearance was waving to fans from the back of a golf cart before the final of the soccer World Cup in Johannesburg's Soccer City stadium in July 2010.
During his retirement, he has divided his time between his home in the wealthy Johannesburg suburb of Houghton, and Qunu, the village in the poor Eastern Cape province where he was born.
The public's last glimpse of him was a brief clip aired by state television in April during a visit to his home by Mr Zuma and other senior ANC officials.