Nelson Mandela was "responding positively" to treatment on Thursday after his latest hospitalisation for a lung infection, the South African presidency said.
"The doctors advise that former president Nelson Mandela is responding positively to the treatment he is undergoing for a recurring lung infection," President Jacob Zuma's office said in a statement.
"He remains under treatment and observation in hospital."
Mandela has been readmitted to hospital with a recurrent lung infection, the presidency said Thursday, urging people to pray for the frail anti-apartheid hero.
The 94-year-old, who has had several recent health scares, was hospitalised "due to the recurrence of his lung infection" just before midnight on Wednesday, President Jacob Zuma's office said in a statement.
"He was conscious" when he was admitted, presidency spokesman Mac Maharaj told AFP.
It marks the second time this month that the Nobel peace laureate has spent the night in hospital and follows a nearly three-week stay in December for the lung infection and for gallstone surgery, after which he was released for home-based care.
The hospital stay earlier this month was for a scheduled medical checkup.
"Doctors are attending to him, ensuring that he has the best possible expert medical treatment and comfort," the presidency said.
Zuma wished "Madiba", as South Africa's first black president is affectionately known, a quick recovery and asked for people around the world to pray for him.
"We appeal to the people of South Africa and the world to pray for our beloved Madiba and his family and to keep them in their thoughts," he said.
"We have full confidence in the medical team and know that they will do everything possible to ensure recovery."
Mandela is adored in South Africa where he is seen as the symbol of the country's peaceful shift to democracy after apartheid.
He has not appeared in public since South Africa's football World Cup final in 2010, six years after retiring.
The ruling African National Congress, the once-banned liberation movement that Mandela led into power, also called for prayers for the beloved former leader.
"During these trying times we wish President Mandela well and for his family to be strong," the party said in a statement.
"We are confident that the treatment will be successful as he is in professional and competent hands," it added.
Hometown villagers 'wish him long life'
The name and location of the hospital were not disclosed, to allow the medical team to focus on their work and to shield the family from the intense media interest.
"We know they are going through a difficult time and we want to ensure that their privacy is maintained," Maharaj said.
Any updates will be based on reports from Mandela's medical team, he said. "I will be guided by the doctors."
Revered at home and abroad, the ailing Mandela has grown increasingly frail away from the public eye.
Mandela's December hospital stay was his longest since he walked free from 27 years of apartheid jail in 1990.
Early last year, he was admitted for a minor exploratory procedure to investigate persistent abdominal pain.
Diagnosed with early-stage tuberculosis in 1988 while imprisoned during the apartheid era, Mandela has long had problems with his lungs.
In 2011, he was hospitalised for two nights for an unnamed acute respiratory infection.
News of his latest ill health was slow to reach Qunu, his rural childhood Eastern Cape village where he had spent recent years before leaving in December.
"Most of the people in the village don't know even that he is in hospital," Zimsile Gamakulu, a local guide and also of the Madiba clan, told AFP .
Villagers "wish him a long life", he said.
"They miss him a lot, especially the older ones," he added.
"They hope that he may come back home."
In February, Zuma said he found Mandela "comfortable and relaxed" and watching television at his Johannesburg home.
"He had the brightest smile," said Zuma.
Earlier this month, his friend and renowned human rights lawyer George Bizos, who defended Mandela during his 1960s treason trial, said the ex-president was aware of current political events but was having some trouble with his memory.
"Unfortunately he sometimes forgets that one or two of them had passed on and has a blank face when you tell him that Walter Sisulu and some others are no longer with us," Bizos said.
Sisulu, a former ANC leader who was Mandela's political mentor, died nearly a decade ago.
Last month, two of his granddaughters released a picture of a smiling Mandela sitting with his youngest great-grandson in an armchair.
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