PRETORIA: Nelson Mandela was back in hospital on Saturday in a “serious but stable” condition, triggering an outpouring of concern for South Africa's beloved national hero at home and across the globe.

Mandela, who turns 95 next month, was admitted to a Pretoria hospital in the early hours suffering a renewed lung infection, his fourth hospital stay in seven months and the third this year.

“The truth of the matter is a simple one. Madiba is a fighter and at his age as long as he is fighting, he'll be fine,” presidency spokesman Mac Maharaj told AFP, using Mandela's clan name.

His wife Graca Machel was with him at the hospital after calling off a trip to London earlier this week.

“This morning when Madiba was hospitalised she accompanied him and she is with him at the hospital,” said Maharaj.

The revered anti-apartheid icon, who has long had problems with his lungs, was once again suffering pneumonia after being admitted to hospital in March for 10 days for the same condition, he said.

“His condition deteriorated to the point where it was found necessary to hospitalise him,” said Maharaj, who served time with Mandela at the apartheid prison on Robben Island.

“But I am told by the doctors that he is breathing on his own, so I think that is a positive side,” he added.

The ruling ANC called on South Africans and people around the world to keep “our beloved statesman and icon, Madiba” in their thoughts and prayers.

The Nobel peace laureate is revered around the world as a symbol of peace and forgiveness after leading South Africa into multi-race democracy as its first black president after decades of apartheid rule.

His readmission to hospital, and the acknowledgement that his condition is serious, sparked concern at home and around the world, as South Africans grappled with their hero's increasing frailty.

“My thoughts are with Nelson Mandela, who is in hospital in South Africa,”tweeted British Prime Minister David Cameron.

“Getting too frequent and concerning, he is 94,” tweeted Zwelinzima Vavi, general secretary of Cosatu, the powerful labour union allied to the ANC.

Mandela was receiving care at his Johannesburg home when the lung problems returned.

He was diagnosed with early-stage tuberculosis in 1988 and also has had treatment for prostate cancer and suffered stomach ailments.

The government has not disclosed where he has been taken, although in the past Mandela has been treated at the Mediclinic Heart Hospital in the capital Pretoria which attracted a media camp on Saturday.

In December 2012, he was hospitalised for 18 days for a lung infection and for gallstones surgery, his longest stay since he walked free from 27 years in jail in 1990.

In March he was admitted for an overnight scheduled check-up before returning later that month for a 10-day stay.

President Jacob Zuma's office said Mandela was “receiving expert medical care and doctors are doing everything possible to make him better and comfortable”.

The presidency wished Mandela a speedy recovery with a request for his and his family's privacy to be respected.

Mandela is still a powerful symbol of peace and unity almost 20 years after taking power. But he has not been seen in public since the World Cup final in July 2010.

After serving just one term in office he turned his energy into the battle against AIDS and resolving conflicts particularly in Africa, before announcing in 2004 at the age of 85 that he was stepping out of the public eye.

In March, Zuma appeared to prepare the nation for the passing of the father of the “Rainbow Nation”.

“In Zulu, when someone passes away who is very old, people say he or she has gone home. I think those are some of the things we should be thinking about,” Zuma told the BBC.

The release of television footage in April showing a frail, distant and unsmiling Mandela being visited at home by leaders of the ruling ANC sparked outrage and accusations that the party was exploiting him.

The African National Congress, facing elections next year, has lost much of its Mandela shine, amid widespread corruption, poverty and poor public services.

Mandela's own family has also been locked in a feud over control of various companies.

“It's sad because he's our icon,” said Faith Mashaba outside the Pretoria hospital.

But there was also growing acceptance of his age.

“I think we must not worry that much about his health,” said Calvin Netshifhefhe, saying it was normal for elderly people to get sick.

“But I think people just love him, they just want to see him alive.”