CAPE TOWN - Visiting US President Barack Obama on Sunday visited Robben Island off Cape Town to pay tribute to his "personal hero" -- ailing former South African president Nelson Mandela.

The visit was part of Obama's three-day official trip to South Africa, his second to the country, but first as US president.

The visit was overshadowed by the health condition of Mandela, who has been hospitalized in a Pretoria hospital for three weeks for a recurring lung infection.

Obama, accompanied by his wife Michael and two daughters, took a helicopter ride to the museum on Robben Island, where Mandela spent 18 years in a prison cell.

The US president refrained from visiting Mandela in Pretoria, but met with Mandela's family members instead.

The White House said the decision was made in accordance with the wishes of Mandela's family. Obama himself said he didn't want to be "obstructive" when Mandela was critically ill in bed.

Mandela's condition remains critical but stable after showing improvement in the past two days, according to Zuma, who issued the latest update on Mandela's health after talks with Obama on Saturday.

The government hoped that Mandela, 94, would be out of hospital soon, Zuma said.

During his visit, Obama repeatedly lauded Mandela's contributions to South Africa and the world at large.

Obama said Mandela managed to transform South Africa and inspire the world.

"Think about 27 years in prison. Think about the hardships and struggles of being away from family and friends.

"Reflecting on his years in prison Nelson Mandela wrote that they were dark moments that tested his faith and humanity but he refused to give up," Obama told an audience in a televised address at the University of Johannesburg Soweto Campus on Saturday.

"We join the people of the world in sending our prayers to Madiba and his family because he still inspires us all," Obama said.

Obama will make his keynote speech at the University of Cape Town later Sunday.