Nelson Mandela back in hospital

Updated at 11:11 a.m. ET

Former South African President Nelson Mandela is responding "positively" to treatment after being taken late Wednesday evening to a hospital due to a recurrence of a lung infection he suffered late last year, the South African government said in a statement Thursday.

"The doctors advise that former President Nelson Mandela is responding positively to the treatment he is undergoing for a recurring lung infection," said the statement from the office of South Africa's president. "He remains under treatment and observation in hospital."

Earlier, presidential spokesman Mac Maharaj said the trip to a hospital in the South African capital of Pretoria was not planned.

"No, this wasn't scheduled, as you will appreciate the doctors do work with a great sense of caution when they are treating (Mandela) and take into account his age," he said. "And so when they found that this lung infection had reoccurred they decided to have him immediately hospitalized so that he can receive the best treatment."

The former president and icon of the democratic movement which ended apartheid in South Africa is 94 years old.

Emma Hurd, reporting for CBS News from Johannesburg, said on "CBS This Morning" Thursday that Mandela was undergoing treatment to clear congestion in his lungs.

Sources close to the family told CBS News that Mandela had been put on oxygen for periods of time Wednesday, and his doctors called an ambulance to the home late in the evening after determining he was experiencing a recurrence of the trouble with his lungs.

"He was being monitored at home very carefully, around the clock. This time, doctors became concerned very late at night and he was taken immediately to hospital," Maharaj told CBS News partner network Sky News, adding that the late-night move was "an indication in itself that the doctors were concerned."

Hurd reports that those close to the frail Nobel laureate famed as the icon of the liberation struggle in South Africa said he has good days and bad days. Mandela has been prone to lung infections ever since he suffered from tuberculosis during his 27 years in prison.

Maharaj said Mandela was conscious as he received treatment on Thursday but could not give any further details.

"We know he has a history of lung problems, from TB to a proneness to infections," said Maharaj, adding that the doctors believed the complications Wednesday night were a "recurrence of those old problems."

"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," South African President Jacob Zuma said in a statement.

Mandela was released from a hospital Dec. 26 after receiving nearly three weeks of treatment for a lung infection and gallstones.

The first picture of Mandela after his discharge wasn't released until early February. At the time, his granddaughters said he was in good health and good spirits.

At his birthday last July, CBS News correspondent Mark Phillips was granted rare access to the party at Mandela's home. The former president appeared to be physically healthy but detached from things going on around him, Phillips reported.

Mandela was admitted again to a hospital in Pretoria for tests in early March. Maharaj told CBS News at the time that the March admission was a scheduled visit for tests commensurate with someone of his age. He was released after that visit on March 10.