JOHANNESBURG - Former South African President Nelson Mandela remains in a critical condition at a Pretoria hospital but "continues to show improvement," the government said on Tuesday.The 95-year-old anti-apartheid hero has spent nearly eight weeks in the hospital for treatment for a recurring lung infection.The government has declined to comment on a weekend report by US broadcaster CBS which said that Mandela had recently undergone a minor operation to unblock a dialysis tube.