Nelson Mandela has once again been admitted to hospital with a lung infection, according to officials in South Africa.
The 94-year-old anti-apartheid leader and former president went to hospital just before midnight.
Presidential spokesman Mac Maharaj acknowledged there was cause for worry, but said the medical specialists treating Mr Mandela were very competent.
"The health has been OK given his age, but the downturn last night - obviously when the lung infection recurs, the doctors will want to do everything possible and make sure that they don't allow the infection to spread, that they arrest it as quickly as possible," Mr Maharaj told eNCA, a South African news channel.
He said there had been a global outpouring of messages expressing concern for Mr Mandela's health.
According to a statement from the office of president Jacob Zuma, Mr Mandela is being treated for a "recurrence of his lung infection" and doctors are ensuring he has the best possible treatment.
Mr Zuma wished Mandela a speedy recovery and urged South Africans and the world to pray for the former leader and his family. Mandela has become increasingly frail in recent years.
Mandela has been admitted to hospital several times since last year, most recently earlier this month when he received what a presidential spokesman described as a "successful" medical test.
"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".
"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," said Mr Zuma.
Mandela spent a night in a hospital and was released on March 10 after a medical test. At that time, presidential spokesman Mac Maharaj said Mandela was "well".
In December, Mandela spent three weeks in hospital as he was treated for a lung infection and had a procedure to remove gallstones.
A year ago, he was admitted to a Johannesburg hospital for what officials initially described as tests but what turned out to be an abdominal complaint. He was discharged days later.
He 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.
Mandela last made a public appearance on a major stage when South Africa hosted the 2010 World Cup.