CAPE TOWN (AP, AFP) - United States President Barack Obama toured South Africa's Robben Island, where former South African President Nelson Mandela spent 18 years in a tiny cell as a political prisoner.

Mr Obama, his wife and two daughters viewed the lime quarry where Mr Mandela toiled during captivity. It was also where the 94-year-old anti-apartheid leader developed the lung problems ailing him today.

The Obamas' tour guide is 83-year-old politician Ahmed Kathrada, who was also held captive at the prison. He told the Obamas the prisoners were forced to work every day.

Mr Obama said he was "deeply humbled" by the visit.

"On behalf of our family, we're deeply humbled to stand where men of such courage faced down injustice and refused to yield," Mr Obama wrote in a message in the visitors book co-signed by his wife Michelle.

"The world is grateful for the heroes of Robben Island, who remind us that no shackles or cells can match the strength of the human spirit."