The South African government says reports that Nelson Mandela has left hospital are incorrect.

A statement said the former president is still in hospital where he has been for three months.

He has been receiving treatment for a lung infection. 

The presidency said reports by some international media that Mandela, 95, had been discharged and returned to his home were "incorrect".

"Madiba is still in hospital in Pretoria, and remains in a critical but stable condition," it said in a statement, using Mandela's clan name.

"At times his condition becomes unstable, but he responds to medical interventions," it added, repeating a medical bulletin it had issued a week ago which had said the ailing Nobel Peace Prize laureate was showing "great resilience".

News of Mandela's hospitalisation in June with a recurring lung infection attracted worldwide attention for the revered statesman, who is admired as a symbol of struggle against injustice and of racial reconciliation.

Mandela celebrated his 95th birthday in hospital on 18 July, showered with tributes from around the world.

He spent nearly three decades in prison before being released and being elected South Africa's first black president in multi-racial elections in 1994 that ended apartheid rule.

Mandela's 27 years in prison under white minority rule included 18 years on the notorious Robben Island penal colony. His lung infection dates back to this time, when he and other prisoners were forced to work in a limestone quarry.