PRETORIA (AFP) - Mr Nelson Mandela remained in a "critical but stable" condition on Monday, the presidency said, more than three weeks after the anti-apartheid hero was hospitalised with a recurring lung infection.

President Jacob Zuma urged South Africans to begin planning for Mandela's 95th birthday on July 18.

"We must all be able to do something good for humanity on this day, in tribute to our former president," he said in a statement.

The update came hours after US President Barack Obama left South Africa without seeing his personal hero, who has been in intensive care since June 8.

Mr Obama spent the weekend paying homage to Mr Mandela, including a trip to the Robben Island prison where the man who would become South Africa's first black president was locked up for 18 years - a visit he said left him "deeply humbled".

Mr Obama also spoke to Mr Mandela's wife Graca Machel by telephone, and met several daughters and grandchildren of the ailing icon 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 acquired immunodeficiency syndrome (Aids) campaigner.