Former South African President Nelson Mandela has been admitted to a hospital with a recurring lung infection, South Africa's government has said.
Mandela, 94, has become increasingly frail in recent years and has been admitted to hospital several times since last year, mostly recently earlier this month when he received what a presidential spokesman described as a "successful" medical test.
Mandela was admitted to a hospital just before midnight on Wednesday "due to the recurrence of his lung infection," the office of President Jacob Zuma said in a statement.
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"Doctors are attending to him, ensuring that he has the best possible expert medical treatment and comfort," the statement said. It appealed "for understanding and privacy in order to allow space to the doctors to do their work".
Zuma wished Mandela a speedy recovery, referring to him affectionately by his clan name, "Madiba".
"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. We have full confidence in the medical team and know that they will do everything possible to ensure recovery," the presidential statement quoted Zuma as saying.
Al Jazeera's Haru Mutasa, reporting from Johannesburg, said many South Africans were very concerned about Mandela's health.
"There is a lot activity on social media - Facebook and Twitter. Many people wishing him well, hoping that he will soon be discharged," Mutasa said.
"Most South African understand the fact that he is an old man, and that he will go in and out of hospital more often than not. There isn't a sense of panic right now," she added.
Mandela spent a night in a hospital and was released on March 10 following a medical test. At that time, presidential spokesman Mac Maharaj said Mandela was "well".
In December, Mandela spent three weeks in a hospital, where he was treated for a lung infection and had a procedure to remove gallstones. A year ago, Mandela was admitted to a Johannesburg hospital for what officials initially described as tests but what turned out to be an acute respiratory infection.
He was discharged days later. He also had surgery for an enlarged prostate gland in 1985.
Under South Africa's white-minority apartheid regime, Mandela served 27 years in prison, where he contracted tuberculosis, before being released in 1990.
He later became the nation's first democratically elected president in 1994 under the banner of the African National Congress, helping to negotiate a relatively peaceful end to apartheid despite fears of much greater bloodshed.
He served one five-year term as president before retiring.
Perceived successes during Mandela's tenure include the introduction of a constitution with robust protections for individual rights and the Truth and Reconciliation Commission, a panel that heard testimony about apartheid-era violations of human rights as a kind of national therapy session.
South Africa still struggles with crime, economic inequality and other social ills.
Mandela last made a public appearance on a major stage when South Africa hosted the 2010 World Cup football tournament.