JOHANNESBURG (AFP) - Mr Nelson Mandela is "unstable" at times, but has shown "great resilience" as he remains in a critical condition in hospital, South Africa's presidency said on Saturday.

"While at times his condition becomes unstable, the doctors indicate that the former president has demonstrated great resilience and his condition tends to stabilise as a result of medical interventions," a statement said.

The ailing 95-year-old anti-apartheid icon was hospitalised on June 8 with a lung infection.

"Doctors are still working hard to effect a turnaround and a further improvement in his health and to keep the former president comfortable," President Jacob Zuma's office added.

On August 11, the presidency said Mr Mandela was making "a slow but steady improvement", but that he remained critically ill.

His youngest daughter Zindzi Mandela said two days earlier that he was able to sit up unaccompanied and was "more alert, more responsive".

Mr Zuma urged South Africans to continue praying for Mr Mandela and to keep the country's first black president constantly in their thoughts.