Zuma visited Mandela in hospital Sunday, and spoke with the medical team caring for the 94-year-old anti-apartheid leader.
“The doctors are doing everything possible to get his condition to improve and are ensuring that Madiba is well-looked after and is comfortable. He is in good hands,” Zuma said in a statement on a government website, using the clan name by which Mandela is widely known in South Africa.
He encouraged South Africans to pray for Mandela, his family and the medical team tending to him.
CBC's Margaret Evans says Zuma's statement represents a genuine cause for concern.
"This is quite a change to be using the word 'critical,'" she reported from Johannesburg. "The presidency has come under some criticism for withholding information, sometimes misleading the media about the state of Nelson Mandela's health and there will be pressure on the South African presidency to be as accurate as possible but, certainly the language we are hearing tonight is quite different and that is why there is concern tonight in South Africa."
The Nobel Peace Prize laureate has been in hospital in Pretoria for over two weeks with what doctors said is a recurring lung infection. It marked his fourth hospital visit within the past six months.
"People are prepared that these may be the final days, weeks and months of his life because he has been in hospital so many times," Evans said.
"But, that said, there are people here who say the man was a fighter and they are asking for a little bit of time and privacy for the Mandela family," she added.
Recent reports from the government and family members indicated Mandela's health was improving. On Thursday, his grandson Ndaba said Mandela was "getting better and better each day and hopefully he'll be coming home soon."
Mandela's respiratory problems in recent years are probably connected to his 27-year imprisonment, when he contracted tuberculosis after working in a prison quarry.
Following his release, he again became the leader of the anti-apartheid struggle, and in 1994 became the first president of a democratic South Africa.