JOHANNESBURG, Aug. 24 (Xinhua) -- South Africa's former President Nelson Mandela's condition becomes "unstable" sometimes, but demonstrates great resilience as a result of medical interventions, said the South African Presidency on Saturday.

The Presidency said in a statement that the ailing icon was "still critical but stable, while at times, his condition becomes unstable."

"The doctors indicate that the former President has demonstrated great resilience and his condition tends to stabilize as a result of medical interventions," the statement said.

"Doctors are still working hard to effect a turnaround and a further improvement in his health and to keep the former President comfortable," added the statement.

President Jacob Zuma once again urged South Africans to continue praying for Mandela and to keep him in their thoughts at all times.

Mandela was rushed to a Pretoria hospital for a recurring lung infection on June 8, making his longest hospital stay since last December. Mandela celebrated his 95th birthday on July 18 in hospital.

On Aug. 9, his daughter Zindzi Mandela said the anti-apartheid icon could manage to sit up, a sign of great improvement in his condition. Her words were the most encouraging news about Mandela's health since his hospitalization.

The Nobel Peace Prize laureate has been suffering from a recurring lung infection, which is the result of tuberculosis developed when he was imprisoned for 27 years during the apartheid period.