Obama 'deeply humbled' by Mandela prison visit

Cape Town - US President Barack Obama said on Sunday he was "deeply humbled" by a visit to Robben Island where the gravely ill former president Nelson Mandela spent 18 years in jail.

"On behalf of our family we're deeply humbled to stand where men of such courage faced down injustice and refused to yield," 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."

Mandela is in a critical but stable condition in Pretoria's Medi-Clinic Heart Hospital, where he is being treated for a recurring lung infection.