JOHANNESBURG - Former South African president Nelson Mandela has been readmitted to hospital with a recurrent lung infection, the presidency said Thursday, urging people to pray for the anti-apartheid hero.

The 94-year-old was hospitalised "due to the recurrence of his lung infection" just before midnight on Wednesday, President Jacob Zuma's office said in a statement.

It is 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 surgery to extract gallstones.

Earlier this month, he spent a night in hospital for a "scheduled medical checkup".

"Doctors are attending to him, ensuring that he has the best possible expert medical treatment and comfort," said the presidency.

Zuma wished "Madiba", as South Africa's first black president is fondly known at home, a quick recovery.

"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."

The name or location of the hospital where Mandela is staying was not disclosed.

Recent health scares

Revered at home and abroad, Mandela has grown increasingly frail away from the public eye with several recent health scares.

His December admission was the Nobel Peace Prize winner's longest hospital stay since he walked free from 27 years of apartheid jail in 1990.

In early 2012, he was admitted for a minor exploratory procedure to investigate persistent abdominal pain.

His lungs have also been a longstanding source of health problems.

In 2011, he was hospitalised for two nights for an unnamed acute respiratory infection.

Mandela was diagnosed with early stage tuberculosis in 1988 while imprisoned during apartheid.

In February, Zuma said he had found Mandela "comfortable and relaxed" and watching television after paying him a visit at his Johannesburg home.

"He had the brightest smile," said Zuma.

Earlier this month, friend and renowned human rights lawyer George Bizos, who defended Mandela during his 1960s treason trial, said Mandela was aware of currents political events but had memory lapses.

"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 told radio's Eyewitness News.

Sisulu, the former leader of the ruling African National Congress (ANC) who was Mandela's political mentor, died nearly a decade ago.

At the beginning of last month, two of his granddaughters released a picture of a smiling Mandela sitting with his youngest great-grandson in an armchair.

It was taken to show his recovery after his December hospitalisation, they said while promoting their new reality show, Being Mandela.

Mandela stepped down after one term as president after taking power after 1994 polls that dealt the final death blow to decades of white minority rule.

He is adored in South Africa where he is seen as the symbol of the divided country's peaceful shift into democracy.

He has not appeared in public since South Africa's Football World Cup final in 2010, six years after retiring.

Rumours of his failing health or even death flare up periodically, forcing the government to issue assurances that all is well.