Painted stones with messages wishing former South African President Nelson Mandela a happy birthday are seen in the garden outside his house in Houghton, Johannesburg, March 28, 2013. [Photo/Agencies]

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

The 94-year-old was hospitalized "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 the Nobel peace laureate has spent the night in hospital and follows a stay of nearly three weeks 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 check-up.

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

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," he said.

"We have full confidence in the medical team, and know they will do everything possible to ensure recovery."

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

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

His admission to a hospital in December was his longest stay since he walked free in 1990 from 27 years in prison under apartheid.

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 hospitalized for two nights for an unnamed acute respiratory infection.

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

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," Zuma said.

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

"Unfortunately, he sometimes forgets that one or two of them have passed on and has a blank face when you tell him that Walter Sisulu and some others are no longer with us," Bizos said on a radio program.

Sisulu, the former secretary-general of the ruling African National Congress who was Mandela's political mentor, died nearly a decade ago.

At the beginning of last month, two of Mandela's 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 hospitalization in December, they said while promoting their new reality show, Being Mandela.

Mandela stepped down after one term as president. Elections in 1994 dealt the final death blow to decades of white minority rule.

In South Africa, he is seen as the symbol of the divided country's peaceful shift to democracy.

He has not appeared in public since the World Cup soccer final in South Africa in 2010, six years after retiring. Rumors of his failing health, or even death, surface periodically, forcing the government to issue assurances that all is well.

AFP-AP