CAPE TOWN (AFP) - US President Barack Obama left South Africa on Monday without seeing his hero Nelson Mandela who has entered a fourth week in hospital where he remains critically ill.

President Obama spent the weekend paying homage to the revered leader, including a trip to the Robben Island prison where the anti-apartheid icon spent 18 years - a visit he said left him "deeply humbled".

President Obama stood in the tiny cell once occupied by Mr Mandela on the windswept outcrop near Cape Town, and took his daughters to the lime quarry where the man who would become South Africa's first black president did back-breaking hard labour.

"Mandela's spirit could never be imprisoned - for his legacy is here for all to see," President Obama said in a speech at the University of Cape Town afterwards.

"Nelson Mandela showed us that one man's courage can move the world. And he calls on us to make choices that reflect not our fears, but our hopes - in our own lives, and in the lives of our communities and our countries," he said.

There has been no update on the health of the 94-year-old Nobel peace laureate since Saturday when South African President Jacob Zuma said he remained "critical but stable". Few details have been released about his condition or treatment.

In an interview with Britain's ITV News broadcast over the weekend, his daughter Zindzi said her father was hanging on.

"We do speak to him all the time. We don't assume he's not with us," she said.

When someone comes into the ward "he opens his eyes", she added.

Mr Mandela's ex-wife Winnie Madikizela-Mandela dismissed as "nonsense" suggestions that "we needed to take a decision to pull the tubes".

Well-wishers continued to stop by at the shrine-like wall of goodwill messages outside the Pretoria hospital where Mr Mandela was admitted on June 8 with a recurring lung infection, although there were fewer visitors than in previous days.

One group of supporters were singing and shouting "Viva Mandela Viva". Mr Jabu Nkosi, a 32-year-old technician at the University of Pretoria, was reading the messages with his two young daughters.

"He's a part of us and we're a part of him. If it wasn't for him, the freedom we have today couldn't have been a success. So we are deeply touched by his condition. We know our presence means a lot to him," he told AFP.

A few candles were burning next to piles of flowers and messages of support from South Africans as well as people from as far afield as France, Greece and Pakistan.

On Saturday, President Obama and his wife Michelle called Mr Mandela's wife Graca Machel, and the president then privately visited several daughters and grandchildren of Mr Mandela, to offer support and prayers.

But he decided against rolling up in his massive entourage at the Pretoria hospital where Mr Mandela lies, worried that he would disturb the peace of the man he has described as a "personal inspiration".

Once branded a terrorist by the United States and Britain, Mr Mandela spent 27 years in prison before walking free from a jail near Cape Town in 1990.

He won South Africa's first fully democratic elections in 1994, forging a path of racial reconciliation during his single term as president, before taking up a new role as a roving elder statesman and leading Aids campaigner.

His influence is palpable on President Obama's tour of Senegal, South Africa and his final stop Tanzania, where he will hold talks with President Jakaya Kikwete and visit the Ubungo power plant, after unveiling a new US$7 billion (S$8.9 billion) programme to boost African electric power networks.

He will also lay a wreath at a memorial to those killed in the US embassy bombing in 1998. His wife Michelle will take part in a First Ladies forum hosted by her predecessor in the role, Mrs Laura Bush.

In a strident call for democratic change and good governance during his speech in Cape Town, the US leader used the political legacy of Mandela and South Africa's emergence from grim years of racist apartheid rule as proof that freedom will ultimately prevail.

"History shows us that progress is only possible where governments exist to serve their people and not the other way around," President Obama said to loud cheers.